Docker build needs to use light weight tags

to get the proper version from GitHub releases.

Only use git describe when docker builds master
use the tag name which is stored in the environment
variable SOURCE_BRANCH.

See:
https://docs.docker.com/docker-cloud/builds/advanced/#override-build-test-or-push-commands

hooks/pre_build

@@ -1,6 +1,16 @@
 #!/bin/bash
 
-version=$(git describe HEAD --abbrev=4)
+
+
+git_version=$(git describe HEAD --tags --abbrev=4)
+
+
+if [ ${SOURCE_BRANCH} == "master" ]
+then
+    version=$git_version
+else
+    version=$(echo ${SOURCE_BRANCH} | sed 's/v//')
+fi
 
 cat << EOF > puppetboard/version.py
 #