set the docker config in a step, not the job's env

The runner context does not exist at job level, so naming it there is a syntax
error and the workflow fails before a step runs. RUNNER_TEMP is the same value
and is an ordinary variable once a step is running.

Co-authored-by: Hanzo Dev <[email protected]>

.github/workflows/image.yml

@@ -31,13 +31,17 @@ jobs:
           - arch: arm64
             pool: linux-arm64
     runs-on: ['${{ matrix.pool }}']
-    env:
-      # Each runner gets its own, because five services share this
-      # machine’s HOME and docker/login-action logs out when a job ends —
-      # taking the credential from whichever job is still pushing.
-      DOCKER_CONFIG: ${{ runner.temp }}/docker
     timeout-minutes: 60
     steps:
+      # One docker config per RUNNER: several runner services share this
+      # machine’s HOME, and docker/login-action logs out in its post step,
+      # so a job that finishes takes the credential from a job still
+      # pushing. Set here rather than in the job env, where the runner
+      # context does not exist and naming it is a syntax error.
+      - name: One docker config per runner
+        run: |
+          d="$RUNNER_TEMP/docker"; mkdir -p "$d"
+          echo "DOCKER_CONFIG=$d" >> "$GITHUB_ENV"
       - uses: actions/checkout@v4
       - uses: docker/login-action@v3
         with:
@@ -59,14 +63,18 @@ jobs:
   merge:
     needs: image
     runs-on: [linux-amd64]
-    env:
-      # Each runner gets its own, because five services share this
-      # machine’s HOME and docker/login-action logs out when a job ends —
-      # taking the credential from whichever job is still pushing.
-      DOCKER_CONFIG: ${{ runner.temp }}/docker
     permissions:
       packages: write
     steps:
+      # One docker config per RUNNER: several runner services share this
+      # machine’s HOME, and docker/login-action logs out in its post step,
+      # so a job that finishes takes the credential from a job still
+      # pushing. Set here rather than in the job env, where the runner
+      # context does not exist and naming it is a syntax error.
+      - name: One docker config per runner
+        run: |
+          d="$RUNNER_TEMP/docker"; mkdir -p "$d"
+          echo "DOCKER_CONFIG=$d" >> "$GITHUB_ENV"
       - uses: docker/login-action@v3
         with:
           registry: ghcr.io