Run opam init in OS base image

[dra27]

Split off from #65

[talex5]

It's live and building images now: https://base-images.ocamllabs.io/

[dra27]

Be very afraid 😈

[kit-ty-kate]

This change just broke opam-health-check as it's using the base image with opam 2.1 and now requires user input to upgrade the ~/.opam directory

Step 19/22 : RUN opam switch create --repositories=alpha,beta,default 4.11.1
 ---> Running in 13933edfa09d
This development version of opam requires an update to the layout of /home/opam/.opam from version 2.0 to version 2.1, which can't be reverted.
You may want to back it up before going further.
Continue? [Y/n] n

it's not a big deal but just so you know for next time.

[dra27]

If opam-health-check’s overriding opam, would it be better for it to be using an image at an earlier stage of the pipeline? Or should there therefore be an earlier stage? Each compiler container having to init opam itself ground my poor mechanical disk to a halt 🙂

[kit-ty-kate]

It's ok I just fixed it in kit-ty-kate/opam-health-check-ng@baaa4fd. It's not too much of a bother to me since I only do this step once.
However I was surprised by the behaviour of opam init there so I opened an issue: ocaml/opam#4409