Use args in ci build

2 jobs for master in 45 seconds (queued for 2 seconds)
Name Stage Failure
failed
push Build
$ docker pull $CI_REGISTRY_IMAGE:latest || true
Error response from daemon: manifest for source.dodis.ch:4577/metagrid-go/elasticsearch:latest not found: manifest unknown: manifest unknown
$ docker build --pull --build-arg elasticsearch_version=7 -t $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA .
Sending build context to Docker daemon 35.33kB
Step 1/3 : ARG elasticsearch_version
Step 2/3 : FROM docker.elastic.co/elasticsearch/elasticsearch:${elasticsearch_version}
manifest for docker.elastic.co/elasticsearch/elasticsearch:7 not found: manifest unknown: manifest unknown
Cleaning up file based variables
ERROR: Job failed: exit code 1