add support for building in sub container

.gitea/workflows/simple-container.yaml

@@ -36,6 +36,11 @@ on:
         required: false
         type: string
         default: ""
+      subdirectory:
+        description: "Subdirectory to use for the build context"
+        required: false
+        type: string
+        default: ""
         
     secrets:
       PASSWORD:
@@ -77,6 +82,8 @@ jobs:
 
       - name: Build container
         run: |
+          [ -n "${{inputs.subdirectory}}" ] && cd ${{inputs.subdirectory}}
+
           # split the extra-args and construct build-arg parameters
           set -- ${EXTRA_ARGS}
           args=""