fix: ${{ env.VAR }} is allowed in container (#1108)

Resolves forgejo/forgejo-actions-feature-requests#62

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<!--URL:https://code.forgejo.org/forgejo/runner-->
- bug fixes
  - [PR](https://code.forgejo.org/forgejo/runner/pulls/1108): <!--number 1108 --><!--line 0 --><!--description Zml4OiAke3sgZW52LlZBUiB9fSBpcyBhbGxvd2VkIGluIGNvbnRhaW5lcg==-->fix: ${{ env.VAR }} is allowed in container<!--description-->
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Reviewed-on: https://code.forgejo.org/forgejo/runner/pulls/1108
Reviewed-by: Mathieu Fenniak <mfenniak@noreply.code.forgejo.org>
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>

act/schema/schema_test.go

@@ -242,7 +242,7 @@ jobs:
     name: Build Silo Frontend DEV
     runs-on: ubuntu-latest
     container:
-      image: code.forgejo.org/oci/node:22-bookworm
+      image: code.forgejo.org/oci/${{ env.IMAGE }}
     uses: ./.forgejo/workflows/${{ vars.PATHNAME }}
     with:
       STAGE: dev

act/schema/workflow_schema.json

@@ -2008,7 +2008,7 @@
     },
     "container": {
       "description": "A container to run any steps in a job that don't already specify a container. If you have steps that use both script and container actions, the container actions will run as sibling containers on the same network with the same volume mounts.\n\nIf you do not set a container, all steps will run directly on the host specified by runs-on unless a step refers to an action configured to run in a container.",
-      "context": ["forge", "forgejo", "github", "inputs", "vars", "needs", "strategy", "matrix"],
+      "context": ["forge", "forgejo", "github", "inputs", "vars", "needs", "strategy", "matrix", "env"],
       "one-of": ["string", "container-mapping"]
     },
     "container-mapping": {