Create UCM docker image

[ChrisPenner]

Adds a docker image for folks to quickly and easily try UCM without installing it on their machine.

Unfortunately this isn't working for some reason, when I run the image I get:

/usr/local/bin/ucm/ucm: line 2:    15 Killed                  "$(dirname "$0")/unison/unison" --runtime-path "$(dirname "$0")/runtime/bin/unison-runtime" "$@"

Not sure if it's a mac codesigning thing or some other arcane thing.

Uploads to our Github Packages repo here:

https://github.com/unisonweb/unison/pkgs/container/unison

TODO:

Once we're happy with it we need to make the image public, which I may need help to do, here's what I see:

Implementation notes

• Adds a new job which builds and pushes a docker image
• Call that job from pre-release and release, assigning latest, vx.y.z, and nightly tags as expected
• It's a multi-platform image, so should work on mac M1

Folks may want to docker run exposing port 8080 so they can use the UI, though I doubt ucm ui will work since xdg-open is in the container.

Test coverage

Nope

Loose ends