diff --git a/.simplecov b/.simplecov
index cfbf20b7..1982cd3d 100644
--- a/.simplecov
+++ b/.simplecov
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 SimpleCov.start do
   add_group 'Library', 'lib'
   add_group 'Specs', 'spec'
diff --git a/Gemfile b/Gemfile
index d65e2a66..f7200f13 100644
--- a/Gemfile
+++ b/Gemfile
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 source 'http://rubygems.org'
 
 gemspec
diff --git a/Rakefile b/Rakefile
index 63645126..6c3318ff 100644
--- a/Rakefile
+++ b/Rakefile
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'bundler/setup'
 
 ENV['PATH'] = "/opt/docker/:#{ENV['PATH']}" if ENV['CI'] == 'true'
diff --git a/docker-api.gemspec b/docker-api.gemspec
index fd62c154..1a131c76 100644
--- a/docker-api.gemspec
+++ b/docker-api.gemspec
@@ -1,4 +1,6 @@
 # -*- encoding: utf-8 -*-
+# frozen_string_literal: true
+
 require File.expand_path('../lib/docker/version', __FILE__)
 
 Gem::Specification.new do |gem|
diff --git a/lib/docker-api.rb b/lib/docker-api.rb
index 72917388..9e99242f 100644
--- a/lib/docker-api.rb
+++ b/lib/docker-api.rb
@@ -1 +1,3 @@
+# frozen_string_literal: true
+
 require 'docker'
diff --git a/lib/docker.rb b/lib/docker.rb
index e0a554b3..973e0d5e 100644
--- a/lib/docker.rb
+++ b/lib/docker.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'cgi'
 require 'multi_json'
 require 'excon'
diff --git a/lib/docker/base.rb b/lib/docker/base.rb
index d439ea8d..cdb4366b 100644
--- a/lib/docker/base.rb
+++ b/lib/docker/base.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # This class is a base class for Docker Container and Image.
 # It is implementing accessor methods for the models attributes.
 module Docker::Base
diff --git a/lib/docker/connection.rb b/lib/docker/connection.rb
index 7537f90e..d678d243 100644
--- a/lib/docker/connection.rb
+++ b/lib/docker/connection.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # This class represents a Connection to a Docker server. The Connection is
 # immutable in that once the url and options is set they cannot be changed.
 class Docker::Connection
diff --git a/lib/docker/container.rb b/lib/docker/container.rb
index a203b3fd..3c0cc044 100644
--- a/lib/docker/container.rb
+++ b/lib/docker/container.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # This class represents a Docker Container. It's important to note that nothing
 # is cached so that the information is always up to date.
 class Docker::Container
diff --git a/lib/docker/error.rb b/lib/docker/error.rb
index ec494799..f0316b58 100644
--- a/lib/docker/error.rb
+++ b/lib/docker/error.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # This module holds the Errors for the gem.
 module Docker::Error
 
diff --git a/lib/docker/event.rb b/lib/docker/event.rb
index 2bfc3ee9..4945bca4 100644
--- a/lib/docker/event.rb
+++ b/lib/docker/event.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # This class represents a Docker Event.
 class Docker::Event
   include Docker::Error
diff --git a/lib/docker/exec.rb b/lib/docker/exec.rb
index eb6d6c68..00394bc9 100644
--- a/lib/docker/exec.rb
+++ b/lib/docker/exec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # This class represents a Docker Exec Instance.
 class Docker::Exec
   include Docker::Base
diff --git a/lib/docker/image.rb b/lib/docker/image.rb
index 0d327d4c..eb5dbc6e 100644
--- a/lib/docker/image.rb
+++ b/lib/docker/image.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # This class represents a Docker Image.
 class Docker::Image
   include Docker::Base
@@ -28,7 +30,7 @@ def push(creds = nil, options = {}, &block)
     repo, tag = Docker::Util.parse_repo_tag(repo_tag)
     raise ArgumentError, "Image does not have a name to push." if repo.nil?
 
-    body = ""
+    body = +""
     credentials = creds || Docker.creds || {}
     headers = Docker::Util.build_auth_header(credentials)
     opts = {:tag => tag}.merge(options)
@@ -119,7 +121,7 @@ class << self
     def create(opts = {}, creds = nil, conn = Docker.connection, &block)
       credentials = creds.nil? ? Docker.creds : MultiJson.dump(creds)
       headers = credentials && Docker::Util.build_auth_header(credentials) || {}
-      body = ''
+      body = +''
       conn.post(
         '/images/create',
         opts,
@@ -168,7 +170,7 @@ def save(names, filename = nil, conn = Docker.connection)
         end
         nil
       else
-        string = ''
+        string = +''
         save_stream(names, {}, conn, &response_block_for_save(string))
         string
       end
@@ -196,7 +198,7 @@ def save_stream(names, opts = {}, conn = Docker.connection, &block)
     def load(tar, opts = {}, conn = Docker.connection, creds = nil, &block)
        headers = build_headers(creds)
        io = tar.is_a?(String) ? File.open(tar, 'rb') : tar
-       body = ""
+       body = +""
        conn.post(
          '/images/load',
          opts,
@@ -267,7 +269,7 @@ def import_stream(options = {}, connection = Docker.connection, &block)
 
     # Given a Dockerfile as a string, builds an Image.
     def build(commands, opts = {}, connection = Docker.connection, &block)
-      body = ""
+      body = +""
       connection.post(
         '/build', opts,
         :body => Docker::Util.create_tar('Dockerfile' => commands),
@@ -288,7 +290,7 @@ def build_from_tar(tar, opts = {}, connection = Docker.connection,
       headers = build_headers(creds)
 
       # The response_block passed to Excon will build up this body variable.
-      body = ""
+      body = +""
       connection.post(
         '/build', opts,
         :headers => headers,
diff --git a/lib/docker/messages.rb b/lib/docker/messages.rb
index dd9a2b75..f72b4bd0 100644
--- a/lib/docker/messages.rb
+++ b/lib/docker/messages.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # This class represents all the messages either received by chunks from attach
 class Docker::Messages
 
diff --git a/lib/docker/messages_stack.rb b/lib/docker/messages_stack.rb
index 2ab85165..a5a0fb73 100644
--- a/lib/docker/messages_stack.rb
+++ b/lib/docker/messages_stack.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # This class represents a messages stack
 class Docker::MessagesStack
 
diff --git a/lib/docker/network.rb b/lib/docker/network.rb
index e30cd912..b9acec86 100644
--- a/lib/docker/network.rb
+++ b/lib/docker/network.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # This class represents a Docker Network.
 class Docker::Network
   include Docker::Base
diff --git a/lib/docker/rake_task.rb b/lib/docker/rake_task.rb
index 30e8836c..b93a4f8c 100644
--- a/lib/docker/rake_task.rb
+++ b/lib/docker/rake_task.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # This class allows image-based tasks to be created.
 class Docker::ImageTask < Rake::Task
   def self.scope_name(_scope, task_name)
diff --git a/lib/docker/util.rb b/lib/docker/util.rb
index 58c3eed2..c8549232 100644
--- a/lib/docker/util.rb
+++ b/lib/docker/util.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'set'
 
 # This module holds shared logic that doesn't really belong anywhere else in the
diff --git a/lib/docker/version.rb b/lib/docker/version.rb
index 15ac493b..fe72bb83 100644
--- a/lib/docker/version.rb
+++ b/lib/docker/version.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Docker
   # The version of the docker-api gem.
   VERSION = '2.3.0'
diff --git a/lib/docker/volume.rb b/lib/docker/volume.rb
index 97ef28bd..686f57c8 100644
--- a/lib/docker/volume.rb
+++ b/lib/docker/volume.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # class represents a Docker Volume
 class Docker::Volume
   include Docker::Base
diff --git a/lib/excon/middlewares/hijack.rb b/lib/excon/middlewares/hijack.rb
index df82cdf5..aff1720c 100644
--- a/lib/excon/middlewares/hijack.rb
+++ b/lib/excon/middlewares/hijack.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Excon
   module Middleware
     # Hijack is an Excon middleware which parses response headers and then
diff --git a/spec/cov_spec.rb b/spec/cov_spec.rb
index 12afb4c9..734e8989 100644
--- a/spec/cov_spec.rb
+++ b/spec/cov_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'spec_helper'
 
 SingleCov.not_covered!
diff --git a/spec/docker/connection_spec.rb b/spec/docker/connection_spec.rb
index 6cb725a9..86291298 100644
--- a/spec/docker/connection_spec.rb
+++ b/spec/docker/connection_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'spec_helper'
 
 SingleCov.covered! uncovered: 12
diff --git a/spec/docker/container_spec.rb b/spec/docker/container_spec.rb
index 4d77e43d..d5c8a34f 100644
--- a/spec/docker/container_spec.rb
+++ b/spec/docker/container_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'spec_helper'
 
 SingleCov.covered! uncovered: 39
diff --git a/spec/docker/event_spec.rb b/spec/docker/event_spec.rb
index 07f03317..0ba6effe 100644
--- a/spec/docker/event_spec.rb
+++ b/spec/docker/event_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'spec_helper'
 
 SingleCov.covered! uncovered: 5
diff --git a/spec/docker/exec_spec.rb b/spec/docker/exec_spec.rb
index b422b133..8e19bda8 100644
--- a/spec/docker/exec_spec.rb
+++ b/spec/docker/exec_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'spec_helper'
 
 SingleCov.covered! uncovered: 5
diff --git a/spec/docker/image_spec.rb b/spec/docker/image_spec.rb
index 66c50337..36b023f5 100644
--- a/spec/docker/image_spec.rb
+++ b/spec/docker/image_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'spec_helper'
 
 SingleCov.covered! uncovered: 16
@@ -464,7 +466,7 @@
     end
 
     context 'with a block capturing create output' do
-      let(:create_output) { "" }
+      let(:create_output) { +"" }
       let(:block) { Proc.new { |chunk| create_output << chunk } }
 
       before do
@@ -537,7 +539,7 @@
     let(:non_streamed) do
       Docker.connection.get('/images/get', 'names' => image)
     end
-    let(:streamed) { '' }
+    let(:streamed) { +'' }
     let(:tar_files) do
       proc do |string|
         Gem::Package::TarReader
@@ -723,7 +725,7 @@
       end
 
       context 'with a block capturing build output' do
-        let(:build_output) { "" }
+        let(:build_output) { +"" }
         let(:block) { Proc.new { |chunk| build_output << chunk } }
         let!(:image) { subject.build("FROM debian:stable\n", &block) }
 
@@ -775,7 +777,7 @@
       end
 
       context 'with a block capturing build output' do
-        let(:build_output) { "" }
+        let(:build_output) { +"" }
         let(:block) { Proc.new { |chunk| build_output << chunk } }
 
         it 'calls the block and passes build output' do
@@ -784,7 +786,7 @@
         end
 
         context 'uses a cached version the second time' do
-          let(:build_output_two) { "" }
+          let(:build_output_two) { +"" }
           let(:block_two) { Proc.new { |chunk| build_output_two << chunk } }
           let(:image_two) { subject.build_from_dir(dir, opts, &block_two) }
 
diff --git a/spec/docker/messages_spec.rb b/spec/docker/messages_spec.rb
index 5867485d..b1398ce8 100644
--- a/spec/docker/messages_spec.rb
+++ b/spec/docker/messages_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'spec_helper'
 
 SingleCov.covered! uncovered: 4
diff --git a/spec/docker/messages_stack.rb b/spec/docker/messages_stack.rb
index dab89e62..48934c05 100644
--- a/spec/docker/messages_stack.rb
+++ b/spec/docker/messages_stack.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'spec_helper'
 
 SingleCov.covered!
diff --git a/spec/docker/network_spec.rb b/spec/docker/network_spec.rb
index a47c5d43..b5947091 100644
--- a/spec/docker/network_spec.rb
+++ b/spec/docker/network_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'spec_helper'
 
 unless ::Docker.podman?
diff --git a/spec/docker/util_spec.rb b/spec/docker/util_spec.rb
index 0824c170..5fcbfb6b 100644
--- a/spec/docker/util_spec.rb
+++ b/spec/docker/util_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'spec_helper'
 require 'tempfile'
 require 'fileutils'
diff --git a/spec/docker/volume_spec.rb b/spec/docker/volume_spec.rb
index 56f3c673..37be216f 100644
--- a/spec/docker/volume_spec.rb
+++ b/spec/docker/volume_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'spec_helper'
 
 SingleCov.covered! uncovered: 1
diff --git a/spec/docker_spec.rb b/spec/docker_spec.rb
index d956e916..b6656d13 100644
--- a/spec/docker_spec.rb
+++ b/spec/docker_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'spec_helper'
 
 SingleCov.covered! uncovered: 8
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index 5fe4e69b..8be80cab 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'bundler/setup'
 
 $LOAD_PATH.unshift(File.join(File.dirname(__FILE__), '..', 'lib'))