score-compose and score-k8s - devcontainer and in CI

[mathieu-benoit]

• make compose-up locally and CI
• make k8s-up locally and CI
• .devcontainer

Notes: still issues with make compose-test and make k8s-test, described below. But for now let's merge this first PR as it's already covering the build backstage container in CI now. And let's tackle the 2 remaining issues in future PRs.

• make compose-test - error: CREATE DATABASE "backstage_plugin_app" - permission denied to create database #74
• make k8s-up - Unable to convert config value for key 'integrations.github[0].apps[0].appId' in 'env' to a number #75

[astromechza]

The first issue is because score-compose's default environment provisioner tries to use the shell environment variables at deploy time. However we can't template those into the mounted file at deploy time. The solution here is generally to set a custom environment provisioner from your makefile and import it at score-compose init time.

The second issue I've cut score-spec/score-compose#219 for.

[mathieu-benoit]

Ready for your review to be merged, thanks!