Allow overriding FILTER_FLAG in clear-docker-cache script

In https://gitlab.com/gitlab-org/gitlab-runner/-/issues/38647 users
would like to reuse the clear-docker-cache to delete object that don't
match the FILTER_FLAG.

Make it overrideable.

Author: Axel von Bertoldi

packaging/root/usr/share/gitlab-runner/clear-docker-cache

@@ -16,7 +16,7 @@ fi
 DOCKER_API_VERSION=$(docker version --format '{{.Client.APIVersion}}')
 DOCKER_CLIENT_VERSION=$(docker version --format '{{.Client.Version}}')
 REQUIRED_DOCKER_API_VERSION=1.25
-FILTER_FLAG='label=com.gitlab.gitlab-runner.managed=true'
+FILTER_FLAG="${FILTER_FLAG:-label=com.gitlab.gitlab-runner.managed=true}"
 
 usage() {
   echo -e "\nUsage: $0 prune-volumes|prune|space|help\n"