Umbrel support

[AaronDewes]

This contains #132. In addition, this PR improves to Dockerfile and introduces GitHub actions to automatically build & push a Docker container, allows to overwrite the qr code host name and running on regtest

This requires 3 secrets to be set in the repo settings:

• DOCKER_CONTAINER_USERNAME: The user or organization that owns the account, the docker container is at $DOCKER_CONTAINER_USERNAME/lndhub
• DOCKER_HUB_USERNAME: The Docker hub username
• DOCKER_HUB_ACCESS_TOKEN: The docker hub access token (used to login)

Once you release & tag a new version, it automatically gets pushed to Docker Hub as USER/lndhub:VERSION and USER/lndhub:latest, all pushes to master are uploaded as USER/lndhub:master.

With these changes, LNDHub is finally ready for Umbrel.

[AaronDewes]

Updated & rebased.

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