Add a docker image for CI

[daamien]

Following a discussion on the discord channel:

Here's a basic docker image with all the requirements to build a PGRX project in a CI job. This saves a lot of CI minutes, especially the cargo pgrx init step which is quite long.

By default, the image is based on the latest version available but it can be overriden with --build-arg PGRX_VERSION=0.11.3

This could be completed with a CI action to build and push the image automatically on dockerhub, like this

https://github.com/daamien/docker-pgrx/blob/latest/.github/workflows/build-push.yaml

This would require to add secrets variables in the Github account for DOCKERHUB_USERNAME and DOCKERHUB_TOKEN

This would also provide an easy way to test against alpha/beta versions by simply pulling any version like this

docker pull pgcentralfoundation/pgrx:0.12.0-alpha.1