CI/CD using GitHub Actions

[erikmd]

This PR closes #358. (It also fixes a version metadata in dune-project)

Note: I guess the CI won't start as soon as I open this PR, because it comes from an external fork. See "checklist" below.

Anyway, the experiments I did in the pfitaxel fork (cf. "dry-run" part below) show this will fix #360 (Docker Hub deployment issue).

To sum up, it replaces Travis CI + Docker Hub automated build with GitHub Actions for both CI and CD, keeping the same semantics as much as possible.

Checklist

@yurug below are the steps I have applied to test this PR; you'll need to apply them on your side before merging the PR:

• 1. Browse https://hub.docker.com/repository/docker/ocamlsf/learn-ocaml/builds/edit and delete all build rules (I guess there are two), then click Save;
• 2. Browse https://hub.docker.com/settings/security and create a new personal access token dedicated to learn-ocaml CI/CD;
• 3. Browse https://github.com/ocaml-sf/learn-ocaml/settings/secrets and create two new secrets (repository secrets):
  • DOCKER_USERNAME := your Docker Hub login
  • DOCKER_PASSWORD := the token from item 2;
• 4. Merge this PR to trigger CI and CD from master.

Dry-run

This GitHub Actions configuration gathers four YAML files (there is a little duplication between deploy-*.yml files but it appears GitHub Action doesn't provide yet ways to "include" a YAML in another one):

• build-and-test.yml - example run
• build-macos.yml - example run
• deploy-master.yml - example run
• deploy-tags.yml - example run

Comparison with Docker Hub automated build

According to the line cat /proc/cpuinfo /proc/meminfo I've added in (the first stage of) the Dockerfile, it is very possible issue #360 showed up because of lack of RAM:

• Docker Hub: 1 processor; memory:

  MemTotal:        3997656 kB
  MemFree:         1341656 kB
  MemAvailable:    3380948 kB
  

• GitHub Actions: 2 processors; memory:

  MemTotal:        7121280 kB
  MemFree:         2074152 kB
  MemAvailable:    6172100 kB
  

[yurug]

Thank you!