Provide container image

[telmich]

Is your feature request related to a problem? Please describe.

At the moment when needing to run this script on a kubernetes cluster, you'll need to build your own image.
At the moment we have created https://hub.docker.com/r/ungleich/rust-synapse-compress-state to solve this problem.

Describe the solution you'd like

It would be great if a github ci/cd job could trigger a new release automatically and publish it under the matrix organisation account.

Describe alternatives you've considered

Everyone building yourself seems to be a bit redundant.

[DMRobertson]

There is CI that looks like it's supposed to push an image to dockerhub, but it seems to be broken due to a lack of credentials: https://github.com/matrix-org/rust-synapse-compress-state/actions/runs/5511223779/jobs/10046474348#step:5:6

[DMRobertson]

Per #117 we need to make DOCKERHUB_USERNAME and DOCKERHUB_TOKEN secrets available.

[foosinn]

Hey, just stumbled over this. What about using simply Github as registry? It's easy to setup with a predefined action