fix: various version bumps & refactor dockerfile

Just some version bumps and small cleanup of the Dockerfile.

• Bumped Node to version 18. The new LTS.
• Put more static versions into variables at the top.
• Removed Terraform 1.0 from the matrix. Maybe remove 1.1 as well? 🤔

---

Various changelogs:

• https://github.com/nodejs/node/blob/main/doc/changelogs/CHANGELOG_V18.md
• https://github.com/docker/buildx/releases/tag/v0.9.1
• https://alpinelinux.org/posts/Alpine-3.16.3-released.html