fix: build docker script to checkf or GHA vars

build_docker.sh

@@ -5,7 +5,8 @@ version=$(git describe --tags --always | sed 's/^v//')
 # note: we may want to extend this to also not tag as latest if working tree is dirty.
 # but i think because of how go bindata works, it probably makes a change in the working tree.
 tag=master
-grep -q "master" .git/HEAD && [[ "$version" != *-* ]] && tag=latest
+BRANCH=${GITHUB_HEAD_REF:-${GITHUB_REF_NAME:-$(git branch --show-current)}}
+[[ "$BRANCH" == "master" ]] && [[ "$version" != *-* ]] && tag=latest
 
 
 docker build --tag=grafana/carbon-relay-ng:$tag .