Fixes build when DOCKER_FROM is set but empty

The DOCKER_FROM is set to an empty value in the push tests.
This expands the check to catch this test case

Author: tobiasge

build.sh

@@ -156,7 +156,9 @@ fi
 ###
 # Determining the value for DOCKER_FROM
 ###
-DOCKER_FROM="${DOCKER_FROM-python:3.7-alpine}"
+if [ -z "$DOCKER_FROM"]; then
+  DOCKER_FROM="python:3.7-alpine"
+fi
 
 ###
 # Variables for labelling the docker image