Improve CentOS6 support

Add a more modern Git in CentOS 6 to make it work with OPAM remote refs.

Support generation in pwd for Dockerfiles

• Dockerfile_distro.generate_dockerfiles goes into the current
  directory instead with each Dockerfile suffixed with the release
  name. There is a new generate_dockerfiles_in_directories
  for the old behaviour.
• Move slow ARM distribution out of the default distro list into
  Dockerfile_distro.slow_distros.
• Add optional ?pin argument to dockerfile_distro generation
  to make it easier to customise version of packages installed.

Cleaner API and ARM improvements

• Rearrange OCaml installation commands to be in Dockerfile instead
  of in Dockerfile_opam (which is now purely OPAM installation).
• Create a ~/.ssh folder with the right permissions in all distros.
• Ensure rsync is installed in all the Debian-based containers.
• Correctly label the ARMv7 containers with the arch=armv7 label.
• Use ppx to build instead of camlp4. Now depends on OCaml 4.02+.

Add cloud solver and ARM/4.03 builds

• Remove redundant apk update from Alpine definition.
• Switch default cloud solver to one dedicated to these images so
  they can updated in sync (the default cloud one is getting hit
  by many bulk build hits in parallel and cannot cope with the load).
• Add distro_of_tag and generate_dockerfile to Dockerfile_distro.
• Add nano to images to satisfy opam pin going interactive.
• Also include 4.03.0 flambda build.
• Add ARMv7hf Raspbian distro (Wheezy and Jessie).

Support for distro generation and many API cleanups

• Add dev-repo metadata to OPAM file.
• Add support for installing the cloud solver for platforms where aspcud is not available.
• Add CMD entrypoints for containers.
• Alpine: add bash in container (requested by @justincormack)
• Debian: correct non-interactive typos and add dialog in container
• Remove onbuild triggers from OPAM containers as it inhibits caching (suggestion via @talex5)
• Include specific Debian versions (v7,8,9) in addition to the stable/unstable streams.
• Add Dockerfile.crunch to reduce the number of layers by combining
  repeated RUN commands.
• Set Debian apt-get commands to noninteractive.
• Add support for Ubuntu 12.04 LTS and also bleeding edge 16.04.
• Add sexplib convertors for Dockerfile.t.
• Add Dockerfile_distro module to handle supported online distributions.
• Add Dockerfile.label to support Docker 1.6 metadata labels.
• Add generate_dockerfiles_in_git_branches to make it easier
  to use Docker Hub dynamic branch support to build all permutations.
• Correctly escape the run_exec, entrypoint_exec and cmd_exec
  JSON arrays so that the strings are quoted.
• Run yum clean after a Yum installation.
• Add support for Alpine Linux.
• Cleanup OPAM build directory to save container space after building from source.
• Remove support for OpenSUSE remotes, as it is no longer maintained.

More flexible OPAM source installation

Add a ?prefix argument to install_opam_from_source to let it be installed in a custom location such as /usr (useful for CentOS).

Add OPAM utility modules

Add Dockerfile_opam and Dockerfile_opam_cmdliner modules with specific rules for managing OPAM installations with Dockerfiles.

Initial public release

Docker is a Linux container manager that can build images
automatically by reading the instructions from a Dockerfile. A Dockerfile is
a text document that contains all the commands you would normally execute
manually in order to build a Docker image. By calling docker build from your
terminal, you can have Docker build your image step-by-step, executing the
instructions successively. Read more at http://docker.com

This library provides a typed OCaml interface to generating Dockerfiles
programmatically without having to resort to lots of shell scripting and
awk/sed-style assembly.

• HTML Documentation: https://avsm.github.io/ocaml-dockerfile
• Source:: https://github.com/avsm/ocaml-dockerfile
• Issues: https://github.com/avsm/ocaml-dockerfile/issues
• Email: [email protected]