woodpecker: add "built by" for docker builds

hopefully works? but untested since it doesn't really matter.

.woodpecker/git-docker.yaml

@@ -5,6 +5,9 @@ when:
 steps:
   - name: build
     image: docker.io/woodpeckerci/plugin-docker-buildx
+    environment:
+        BUILTBY:
+            from_secret: BUILT_BY
     settings:
         username:
             from_secret: DOCKER_USERNAME

.woodpecker/stable-docker.yaml

@@ -5,6 +5,9 @@ when:
 steps:
   - name: build
     image: docker.io/woodpeckerci/plugin-docker-buildx
+    environment:
+        BUILTBY:
+            from_secret: BUILT_BY
     settings:
         username:
             from_secret: DOCKER_USERNAME