feat: Add CLI option learn-ocaml build --build-dir=[./_learn-ocaml-build]

[erikmd]

(…where exercises are precompiled).

Cc @AltGr FYI:

• I successfully tested the new CLI option on my machine;
• I am currently building the Docker image to test the new entrypoint as well;
• Given the patch is relatively straightforward (in particular, rsync did not seem required),
  if you don't object now, I plan to merge it as soon as the CI is green and my local Docker tests pass as well,
  then prepare the release notes (just reordering the CHANGES items in chore(master): release 1.0.0 #572),
  and then pass the baton to you! for merging the release-please PR and taking care of the release announcement.

Kind regards.

Description

• Kind: enhancement

Motivation:

• in current master, the exercises were always precompiled in-place;
• this conflicted with the documented workflow (option "ro"):
  • https://ocaml-sf.org/learn-ocaml/howto-deploy-a-learn-ocaml-instance.html
  • https://ocaml-sf.org/learn-ocaml/howto-setup-exercise-development-environment.html
• this new option makes it possible to copy and precompile exercises apart by default
  (using the learn-ocaml binary, or the official Docker image).

Examples:

• learn-ocaml build --repo=demo-repository
• learn-ocaml build --repo=demo-repository serve
• learn-ocaml build --repo=demo-repository serve --replace # or
• learn-ocaml build --repo=demo-repository --build-dir=demo-repository # to retrieve the previous behavior.

Checklist

• Read the CONTRIBUTING.md guide and:
  • Use Atomic Commits so each commit gathers a single logical change
  • Use Conventional Commits regarding commit messages (needed by our release toolchain)
• Add/update tests
• Add/update documentation
• …

Note to maintainers

• Read this wiki page.
• Make sure the PR has a milestone.
• Assign yourself before merging.
• Either do a regular merge:
  • for PRs containing several commits following conventional-commits,
  • or for PRs containing 1 commit shared with a later PR (to preserve the SHA1)
• Or do a squash-merge:
  • for PRs containing only 1 commit (not shared with a later PR),
  • or for PRs containing several commits that need not be kept in the history;
  • → Update the commit message header with a conventional-commit type,
  • → Add a footer Close #… if a related issue exists.

[erikmd]

Hi @AltGr ! Good news, the feature is OK.
However after extensive testing, I noted the following:

• externalizing the build-dir in a volume looks unneeded
• indeed, if the build-dir is shared, but the container is deleted, the www is deleted as well, and the exercices are rebuilt anyway
• moreover, adding another volume adds more noise - more work needed regarding docker run -v args

I need to be away from keyboard for ~2h but I'll push another commit to fix this afternoon (and squash the PR in the end).

Meanwhile, to be on the safe side, I'll tweak another detail in this PR (if the build-dir is "learn-ocaml-build" or "./learn-ocaml-build", then the learn-ocaml-build/exercises subfolder is "rm -fr" to be sure that there is no obsolete leftover (I know this is a bit aggressive, but let's see this as a poor man's "rsync --delete-delay"))

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Finally, I notice that the "--replace" feature could be improved regarding Docker side, but this will deserve another PR (after 1.0.0)