Pass tag to dapper container

Drone does a git clone with `--no-tags` so the normal version script
does not work.

scripts/version

@@ -5,7 +5,7 @@ if [ -n "$(git status --porcelain --untracked-files=no)" ]; then
 fi
 
 COMMIT=$(git rev-parse --short HEAD)
-GIT_TAG=$(git tag -l --contains HEAD | head -n 1)
+GIT_TAG=${DRONE_TAG:-$(git tag -l --contains HEAD | head -n 1)}
 
 if [[ -z "$DIRTY" && -n "$GIT_TAG" ]]; then
     VERSION=$GIT_TAG