Use the built docker image, so it doesn't build the docker image on every run

[michael-bud]

No description provided.

[pascalgn]

I assume this is still a draft? Because when I try it, I get

##[error]'docker.pkg.github.com/pascalgn/automerge-action/automerge-action' should be either '[path]/Dockerfile' or 'docker://image[:tag]'.

[michael-bud]

@pascalgn Nope, just a mistake (the change is untested). Apologies, should be fixed now.

[pascalgn]

No, this still isn't working:

Are you aware of #24? Because I think this (still) isn't supported

[pascalgn]

I think this isn't ready yet. If you have additional input or comments, feel free to reopen!