Testing docker image

[sobolevn]

Right now docker images are not tested.
It might be a problem.

I suggest to add a special action that runs separately and check that docker image works correctly.
We can run only a very simple checks, not all test suite. For example:

1. Port is available, response is correct
2. Mix.install works

Refs #543
Refs #552

[jonatanklosko]

Since this is a reproducible image, the only moving part is Livebook itself, and that we do test. The difference from the mix setup is that we package it into a release, but with that we could at most do some http-request level testing and I don't think this would cover much. Given that Livebook is a dev tool, it seems perfectly effective to rely on the users reporting any discrepancies (which, as said, are not very likely assuming the initial image works fine). @josevalim wdyt?

If you have any particular concern/scenario in mind or I missed the point let me know :)

[josevalim]

I agree with @jonatanklosko. The image is going to be mostly static, so I think the odds of messing it up is pretty low to the point I am comfortable with not having image tests.

[sobolevn]

Ok! I am going to close this. We can always reopen the task and add tests if needed. 👍