Use outputs

.github/workflows/build_docker.yml

@@ -45,6 +45,13 @@ jobs:
       with:
         images: ghcr.io/${{ github.repository }}
 
+    - name: Generate build variables
+      id: prep
+      run: |
+        echo "built_at=$(date +%Y-%m-%dT%H:%M:%S)" >> $GITHUB_OUTPUT
+        echo "go_version=$(go version | cut -d ' ' -f 3)" >> $GITHUB_OUTPUT
+        echo "git_commit=$(git rev-parse HEAD)" >> $GITHUB_OUTPUT
+
     - name: Build and push Docker image
       uses: docker/build-push-action@v5
       with:
@@ -56,9 +63,9 @@ jobs:
         cache-from: type=local,src=/tmp/.buildx-cache
         cache-to: type=local,dest=/tmp/.buildx-cache
         build-args: |
-          BuiltAt=$(date +%Y-%m-%dT%H:%M:%S)
-          GoVersion=$(go version)
-          GitCommit=$(git rev-parse HEAD)
+          BuiltAt=${{ steps.prep.outputs.built_at }}'
+          GoVersion=${{ steps.prep.outputs.go_version }}'
+          GitCommit=${{ steps.prep.outputs.git_commit }}'
           Version=${{ steps.version.outputs.version }}
 
     - name: Extract metadata (tags, labels) for Docker