chore: add assertions for the forge context and use it instead of github where relevant (#748)

Refs forgejo/act#37

Reviewed-on: https://code.forgejo.org/forgejo/end-to-end/pulls/748
Reviewed-by: Michael Kriese <michael.kriese@gmx.de>
Co-authored-by: Earl Warren <contact@earl-warren.org>
Co-committed-by: Earl Warren <contact@earl-warren.org>

actions/example-docker-action/.forgejo/workflows/test.yml

@@ -9,11 +9,11 @@ jobs:
       #
       - uses: https://data.forgejo.org/forgejo/test-setup-forgejo-docker@main
         with:
-          args: ${{ github.workspace }}/SOMEFILE
+          args: ${{ forge.workspace }}/SOMEFILE
       #
       # ... which then also exists in the job workspace
       # because both docker containers are automatically
       # sharing the volume that contains it.
       #
       - run: |
-          test -f ${{ github.workspace }}/SOMEFILE
+          test -f ${{ forge.workspace }}/SOMEFILE