Get GraalVM container images from GitHub packages

[raboof]

Since they have disappeared from Docker Hub: https://www.graalvm.org/docs/getting-started/#graalvm-community-container-images

May need an update to CircleCI to set up credentials to log in to GitHub, let's see.

[pvlugter]

I've updated the GITHUB_TOKEN env var to the token currently used for cloudstatebot, and reverted back the latest commits. But realising that the secrets will of course not be available to pull requests from forks. If we need to authenticate with secrets, then we'll likely need a different approach here, as we shouldn't enable secrets for forked pull requests.

[pvlugter]

Ok, looking at this more closely. So the older version of Graal that we use is published to docker.pkg.github.com/graalvm/container/community but docker.pkg.github.com requires authentication for all pulls. The newer Graal docker images are at ghcr.io/graalvm/graalvm-ce, which allows unauthenticated pulls, but doesn't have the older version that we're using. Trying an upgrade to 20.3.1 and it doesn't build — will need changes. So possibilities as I see it:

• create some public guest credentials that can go into CI build directly
• republish the docker image for the older version to another repository
• update to a newer Graal version so we can pull from ghrc.io instead (this is what we'll want to do at some point anyway)

[pvlugter]

I'm trying a minimal Graal update in #519

[pvlugter]

Closing this one, with #519 passing tests.

[raboof]

Great, thx!