Use pre-baked docker image in deploy action

[oharsta]

In the current github deploy action various packages are installed in the docker image. If we use a pre-baked docker image (see https://github.com/OpenConext/OpenConext-BaseContainers/) then the deploy task can be speed up by ~12 minutes. Also the deploy task can start before all the tests are run. The actual push of the image can wait for the tests to succeed