InvalidDefaultArgInFrom: Default value for ARG results in empty or invalid base image name

[antoineco]

I noticed the following warning today while spawning a new stack:

=> WARN: InvalidDefaultArgInFrom: Default value for ARG docker.elastic.co/elasticsearch/elasticsearch:${ELASTIC_VERSION} results in empty or invalid base image name (line 4)

Nothing critical, but we should probably set a default value for all build ARGs to avoid scaring users.

docker-elk/elasticsearch/Dockerfile

Line 1 in fbf3041

ARG ELASTIC_VERSION

Ref. https://docs.docker.com/reference/build-checks/invalid-default-arg-in-from/