tox.ini (docker): Only append to Dockertags if BUILD_TAG is set

Author: Matthias Köppe

tox.ini

@@ -778,7 +778,9 @@ commands =
     docker:            else \
     docker:              PUSH_TAGS=$(echo $BUILD_IMAGE:$BUILD_TAG; for tag in {env:EXTRA_DOCKER_TAGS:}; do echo "$BUILD_IMAGE:$tag"; done); \
     docker:            fi; \
-    docker:            echo $BUILD_IMAGE:$BUILD_TAG >> {envdir}/Dockertags; \
+    docker:            if [ -n "$BUILD_TAG" ]; then \
+    docker:              echo $BUILD_IMAGE:$BUILD_TAG >> {envdir}/Dockertags; \
+    docker:            fi; \
     docker:            if [ x"{env:DOCKER_PUSH_REPOSITORY:}" != x -a x"$PUSH_TAGS" != x ]; then \
     docker:              echo Pushing $PUSH_TAGS; \
     docker:              for tag in $PUSH_TAGS; do \