Updating alpine and gem versions

Dockerfile

@@ -1,9 +1,9 @@
-FROM alpine:3.3
+FROM alpine:3.8
 MAINTAINER [email protected]
 
 RUN apk update && apk --update add ruby ruby-irb ruby-io-console tzdata ca-certificates
 
-ADD Gemfile /app/  
+ADD Gemfile /app/
 ADD Gemfile.lock /app/
 
 RUN apk --update add --virtual build-deps build-base ruby-dev \
@@ -12,8 +12,8 @@ RUN apk --update add --virtual build-deps build-base ruby-dev \
     && bundle install \
     && apk del build-deps
 
-ADD . /app  
-RUN chown -R root:root /app  
+ADD . /app
+RUN chown -R root:root /app
 
 WORKDIR /app
 

Gemfile

@@ -1,4 +1,5 @@
 source 'https://rubygems.org'
 
-gem 'midi-smtp-server', '~> 2.1.2'
+gem 'midi-smtp-server', '~> 2.2.4'
 gem 'mailgun-ruby', '~> 1.1.0'
+gem 'json'

Gemfile.lock

@@ -1,31 +1,33 @@
 GEM
   remote: https://rubygems.org/
   specs:
-    domain_name (0.5.20160826)
+    domain_name (0.5.20180417)
       unf (>= 0.0.5, < 1.0.0)
     http-cookie (1.0.3)
       domain_name (~> 0.5)
-    json (1.8.3)
-    mailgun-ruby (1.1.0)
-      json (~> 1.8)
-      rest-client (~> 1.6)
-    midi-smtp-server (2.1.2)
-    mime-types (2.99.2)
+    json (2.1.0)
+    mailgun-ruby (1.1.11)
+      rest-client (~> 2.0.2)
+    midi-smtp-server (2.2.4)
+    mime-types (3.2.2)
+      mime-types-data (~> 3.2015)
+    mime-types-data (3.2018.0812)
     netrc (0.11.0)
-    rest-client (1.8.0)
+    rest-client (2.0.2)
       http-cookie (>= 1.0.2, < 2.0)
-      mime-types (>= 1.16, < 3.0)
-      netrc (~> 0.7)
+      mime-types (>= 1.16, < 4.0)
+      netrc (~> 0.8)
     unf (0.1.4)
       unf_ext
-    unf_ext (0.0.7.2)
+    unf_ext (0.0.7.5)
 
 PLATFORMS
   ruby
 
 DEPENDENCIES
-  mailgun-ruby
-  midi-smtp-server
+  json
+  mailgun-ruby (~> 1.1.0)
+  midi-smtp-server (~> 2.2.4)
 
 BUNDLED WITH
-   1.11.2
+   1.16.3