github actions: build OCI images that contain both amd64 and armv7

[nodiscc]

• ref. https://docs.docker.com/engine/reference/commandline/buildx_build/#platform
• ref. https://docs.docker.com/build/ci/github-actions/multi-platform/
• replaces Build Docker images that contain both amd64 and /arm/v7 #1496
• make docker image name configurable through CI variables for easier testing

[nodiscc]

Rebased, PR description updated. Verified that the resulting image contains both amd64 and arm/v7 builds:

$ docker buildx imagetools inspect ghcr.io/nodiscc/shaarli:latest
Name:      ghcr.io/nodiscc/shaarli:latest
MediaType: application/vnd.docker.distribution.manifest.list.v2+json
Digest:    sha256:c7233e7e11f20c37709150255f66dcb7aa23757a4df7eb258feae9c1c87a42aa
           
Manifests: 
  Name:      ghcr.io/nodiscc/shaarli:latest@sha256:c4c74907f18c35b50d30b22e3219c0b8aaee5cb0e943162fedf9c7133713c7aa
  MediaType: application/vnd.docker.distribution.manifest.v2+json
  Platform:  linux/amd64
             
  Name:      ghcr.io/nodiscc/shaarli:latest@sha256:2f68dde7e685ca41250cafb7a3d084aeea80f057e28710507b18ed61015a5da2
  MediaType: application/vnd.docker.distribution.manifest.v2+json
  Platform:  linux/arm/v7

I made the image name configurable through Github Actions CI variables/secrets, so I will now set DOCKER_IMAGE = shaarli/shaarli in https://github.com/shaarli/Shaarli/settings/secrets/actions.

I will now close #1496 since this PR replaces it. I have no way to test if the resulting image actually works (currently no ARM machine to run it, I will try to create an ARMv7 VM with libvirt/qemu but this might take some time). Maybe @cerebrate or other users with an ARM machine would be able to test ghcr.io/shaarli/shaarli:latest once this PR is merged?

[cerebrate]

Happy to do that testing.

...incidentally, since it's got a lot more popular since I was first noodling about with arm devices, maybe it's time to add arm64 to the build matrix also?

[ArthurHoaro]

I made the image name configurable through Github Actions CI variables/secrets, so I will now set DOCKER_IMAGE = shaarli/shaarli in shaarli/Shaarli/settings/secrets/actions.

Wouldn't it make sense to use a variable instead of a secret, since it's not sensitive information?

...incidentally, since it's got a lot more popular since I was first noodling about with arm devices, maybe it's time to add arm64 to the build matrix also?

Agreed, and I happen to have a brand new arm64 chip to test it!

Current behavior:

> docker pull ghcr.io/shaarli/shaarli
Using default tag: latest
latest: Pulling from shaarli/shaarli
no matching manifest for linux/arm64/v8 in the manifest list entries

[cerebrate]

@nodiscc Looks to be running fine on my 32-bit ARM machine (Raspberry Pi).

[nodiscc]

Wouldn't it make sense to use a variable instead of a secret, since it's not sensitive information?

A plain variable would be more appropriate, but I used secrets since we already use them - this way the syntax stays consistent (https://docs.github.com/en/actions/learn-github-actions/variables vs https://docs.github.com/en/actions/security-guides/encrypted-secrets), and everything is immediately visible on Secrets and variables > Actions - I prefer consistency to correctness :) But feel free to change it.