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).