Check docker warnings

[fzipi]

Latests builds are showing these warnings

 5 warnings found (use docker --debug to expand):
 - InvalidDefaultArgInFrom: Default value for ARG httpd:${HTTPD_VERSION}-alpine results in empty or invalid base image name (line 69)
 - SecretsUsedInArgOrEnv: Do not use ARG or ENV instructions for sensitive data (ENV "PROXY_SSL_CERT_KEY") (line 77)
 - SecretsUsedInArgOrEnv: Do not use ARG or ENV instructions for sensitive data (ENV "SSL_CERT_KEY") (line 77)
 - InvalidDefaultArgInFrom: Default value for ARG httpd:${HTTPD_VERSION}-alpine results in empty or invalid base image name (line 3)
 - InvalidDefaultArgInFrom: Default value for ARG httpd:${HTTPD_VERSION}-alpine results in empty or invalid base image name (line 49)

Take a look and see if they can be fixed.

[theseion]

The InvalidDefaultArgInForm are ok. I would expect the build to fail if the version is missing.