Fix development version of Docker image

validation/Dockerfile

@@ -134,7 +134,7 @@ RUN set -e pipefail; \
 # switch to development version via build-arg
 ARG ADD_DEV
 RUN set -e pipefail; \
-    if test xYES == x`echo ${ADD_DEV} | tr '[:lower:]' '[:upper:]'`; then \
+    if test xYES = x`echo ${ADD_DEV} | tr '[:lower:]' '[:upper:]'`; then \
       apk add bash emacs gcc build-base; \
       /usr/local/bin/python -m pip install pylint black; \
       apk del gcc build-base; \