ci: add github-actions to publish multi-arch container

[oliverpool]

As proposed in #10

Instead of using the Dockerfile, this leverages the dotnet publish command to build multi-arch (x64 and arm64) container images.

• on push to main, the container is built for x64 (smoke test, to ensure that the docker build is not broken)
• on tag, the container is built and published for x64 and arm64 (multi-image arch, so the callers don't have to worry about it)

This works fine, but I had to adjust 2 aspects:

Sqlite DB path change

/app is owned by an user different from the user running the app, so the sqlite db can't be created there. I moved it to /home/app (this also allows consumers to set a volume for the whole /home/app without messing with the dll).

Https -> http

Generating self-signed certs in a previous step is a bit hacky (self-signed certs are easy to misuse and the expiry date is like a time-bomb).

I think that https certs should be managed outside of the docker container (reverse-proxy for instance).

Since dotnet defaults to rootless containers, the app now accepts http requests on port 8080.

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You can try the current state on https://github.com/oliverpool/OAuch/pkgs/container/oauch

docker  run -p 8080:8080 ghcr.io/oliverpool/oauch:main

[oliverpool]

@pieterphilippaerts let me know what you think!