Allow PRs to build images

[puddly]

This PR makes the container building step optional, allowing PRs that modify both to be done in two stages:

1. Make a PR that updates Dockerfile
2. Once merged, a new container will be built and pushed. This can then be used by subsequent PRs.

Since no write access is granted to the container for non-members, it can be used by normal PRs within CI (after approval).

[puddly]

Should be good now. Took a very long time to figure out that pushing changes to a fork triggers pull_request via CI on this repo and simultaneously push on my fork. Both now pass:

• https://github.com/puddly/silabs-firmware-builder/actions/runs/8917502964
• https://github.com/NabuCasa/silabs-firmware-builder/actions/runs/8917503273

[puddly]

I've brought build times down to about 5 minutes for the base Docker container by downloading the release ZIPs instead of checking out the SDK repos with Git. Firmwares build in about 30s each (in parallel), after 2 minutes is spent pulling the above container.

Tested with an unreleased PR bumping Gecko SDK: https://github.com/puddly/silabs-firmware-builder/actions/runs/8940084401

[agners]

Nice!