[CI] - Put all CI steps on GITHUB_OUTPUT (remove GITHUB_ENV)

[dev-dull]

CI uses a mix of GITHUB_ENV and GITHUB_OUTPUT. The use of GITHUB_ENV is leftover from initial development. It should be possible to consolidate on GITHUB_OUTPUT.

PyTerraBackTyl/.github/workflows/docker-build.yaml

Line 25 in 3f2e781

# TODO: going to both ENV and OUTPUTS is probably redundant