Use a custom docker image

[gdamjan]

Use a custom docker image to run the cron instead of installing planet and deps all the time.

A separate github action can be added to the same repo, reacting on changes to Dockerfile and that publishes the docker image. The use that image in the deploy action.

see https://github.com/gdamjan/TidyTuesdayAtKIKA/blob/master/.github/workflows/docker-image.yml for insipiration.

[gdamjan]

Lets do this after we can use github hosted docker images in github actions.

Currently only public images can be used on github actions, and github doesn't host public images.