Address docker warnings

[lloeki]

There are a bunch of warnings in Docker output, like:

X warnings found (use docker --debug to expand):
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line XX)
 - SecretsUsedInArgOrEnv: Do not use ARG or ENV instructions for sensitive data (ARG "REPRO_RUN_KEY") (line XX)

Note: REPRO_RUN_KEY is not a secret, looks like a naive "_KEY means secret" heuristic.