Docker package build

[ardocrat]

Added docker image build on push at master branch.

• Made a single Dockerfile to use single image for testnet and mainnet
• Added support for --testnet argument in the end of grin server run command to use single image at compose files.
• Publish to ghcr.io on build

Release member from Mimblewimble should create Secrets for repo (https://github.com/mimblewimble/grin/settings/secrets/actions):

• GHCR_USERNAME with name of organisation (mimblewimble)
• GHCR_TOKEN from here https://github.com/settings/tokens/new (choose write:packages, read:packages permissions).

After build need to check that package is public at https://ghcr.io/mimblewimble/grin and change visibility if needed here https://github.com/orgs/mimblewimble/packages/container/grin/settings.

[DavidBurkett]

The duplicate --testnet/--usernet additions to server run are a little bit weird. I don’t think we actually need to modify the Grin CLI to support this Docker use case.

Right now the need for subcommand-level flags is coming from the Dockerfile using:

ENTRYPOINT ["grin", "server", "run"]

Because Docker only appends args, that forces things like --testnet to appear after server run.

An alternative that avoids changing the CLI at all would be to restructure the Dockerfile like this:

ENTRYPOINT ["grin"]
CMD ["server", "run"]

That preserves the existing, canonical CLI structure and lets users do:

docker run grin
# -> grin server run

docker run grin --testnet server run
# -> grin --testnet server run

This still works cleanly with docker-compose via command: overrides. This keeps chain selection where it already lives (command-level), avoids duplicating flags on subcommands, and keeps the Rust CLI unchanged.