GitHub actions: externalize build matrix

[giorio94]

Description

This PR modifies the build workflow, to enable a more granular control on which containers are build depending on the triggering event. Specifically, the matrix definition is moved to an external json file, that is read and optionally filtered by a custom action during the pre-processing step. The current configuration ensures that all the core images are always built, while user environments are built only in case of stable releases and if the /deploy-staging command is triggered with an appropriate flag.

How Has This Been Tested?

• Checking the correct functioning of the workflow on a personal fork.

[kingmakerbot]

Hi @giorio94. Thanks for your PR.

I am @kingmakerbot.
You can interact with me issuing a slash command in the first line of a comment.
Currently, I understand the following commands:

• /rebase: Rebase this PR onto the master branch
• /merge: Merge this PR into the master branch
• /hold: Adds hold label to prevent merging with /merge
• /unhold: Removes the hold label to allow merging with /merge
• /deploy-staging: Deploy a staging environment to test this PR
• /undeploy-staging: Manually undeploy the staging environment

Make sure this PR appears in the CrownLabs changelog, adding one of the following labels:

• kind/breaking: 💥 Breaking Change
• kind/feature: 🚀 New Feature
• kind/bug: 🐛 Bug Fix
• kind/cleanup: 🧹 Code Refactoring
• kind/docs: 📝 Documentation

[giorio94]

/rebase

[giorio94]

/merge