Refresh distribution support (Debian, Alpine, OCaml)

• Mark OCaml 4.05.0 as a released stable version.
• Remove the Alpine 3.5 camlp4 hack as it has been fixed in a
  point release upstream.
• Add minimum constraint on sexplib in build rules (#6 reported by @smondet)
• Add support for Alpine 3.6 and Debian 10 (Buster).
• Bump the most recent Debian Stable to Debian 9.
• Bump the most recent Alpine to Alpine 3.6.
• Add OCaml 4.04.2 as the most recent compiler

Multistage build support, and split opam packages

• Add support for multistage builds
  to the from, add, and copy commands.

There are also backwards incompatible changes to the package layout:

• Split up OPAM packages into dockerfile and dockerfile-opam.
  The latter contains the OPAM- and Linux-specific modules, with
  the core DSL in dockerfile.
• Port to jbuilder.

Add OPAM2 sandboxing wrappers and update Ubuntu/Fedora

• Add OCaml 4.04.1 to the stable released set.
• Add Ubuntu 17.04 and Fedora 25 to the distribution list.
• Setup OPAM2 wrappers in containers. This will enforce Linux
  namespaces upon building and installing the packages, preventing
  them from doing network access when they shouldn't or writing files
  where they shouldn't (#1 from @AltGr). These are not activated
  by default and are present in /etc/opamrc.userns in the relevant
  OPAM2 containers.

Support OCaml 4.06 and Alpine 3.5 better

• Register 4.06.0 as a trunk compiler revision.
• Correctly install aspcud in all Alpine 3.5 containers.

Support for OPAM2 beta1 and OCaml 4.06.0dev

• Bump latest stable OCaml to 4.04.0.
• Add OCaml 4.06.0dev into the build matrix.
• Support latest OPAM 2.0beta release.
• Bump the "latest" distro tags to Alpine 3.5 and OpenSUSE 42.2.

Alpine 3.5, Ubuntu 16.10 and remote solver improvements

• Remove support for ARM variants from the default distribution
  list. They will come back as explicitly supported multiarch
  targets, instead of the current qemu builds that are mixed up
  with x86_64 targets.
• Always install OPAM from source on Alpine until upstreaming
  is complete.
• Register 4.04 as a mainline compiler as well (fixes OPAM2).
• Add support for Alpine 3.5 and OpenSUSE 42.2, and promote
  the Alpine:latest images to Alpine 3.5.
• Do not install camlp4 by default in distributions.
• Refresh aspcud remote proxy with url-escaping fixes
  (via @OCamlPro-Henry in ocaml/opam#2809)
• Add Ubuntu 16.10 to the built-distros list.

Module rearrangements

• Move Dockerfile.Linux to a separate Dockerfile_linux
  module, in preparation for Dockerfile_windows soon.
• Avoid using ppx annotations for sexp in the interface
  files, since this breaks ocamldoc.
• Add Dockerfile.pp for Format-style output.

Fedora and OPAM2 support, and safe-string

• Port to build using topkg and remove _oasis.
• Support -safe-string mode.
• Install xz into base Fedora and other RPM distros.
• Expose a Linux.RPM.update to force a Yum update.
• Install openssl as a dependency for OPAM2.

Latest OPAM2 support and update OpenSUSE

• Support OPAM 2 better with explicit compiler selection.
• Correctly install ocamldoc in system OpenSUSE container.

OPAM2 support, OpenSUSE/Ubuntu and distro refresh

• Multiarch: Add Alpine 3.4 and Alpine/ARMHF 3.4 and deprecate Raspbian 7.
• Add OpenSUSE/Zypper support and add OpenSUSE 42.1 to the default distro build list.
• Add Ubuntu 16.10 to the distro list, and remove Ubuntu 15.10 from default build list now that 16.10 LTS is available.
• Add Fedora 24 and make it the alias for Fedora stable. Also install redhat-rpm-config which is needed for pthreads.
• Add an extra arg the Dockerfile_distro matrix targets to add more distros to the mix, such as Raspbian.
• Support multiple OPAM versions in the matrix generation, to make testing OPAM master easier.
• Always do an rpm --rebuilddb before a Yum invocation to deal with possible OverlayFS brokenness.
• Support opam_version to distro calls to build and install the latest version of OPAM2-dev.
• Add xz into Alpine containers so that untar of those works.
• Expose the development versions of OCaml compilers.