Environment variable for log level

[taladar]

It would be helpful to debug issues with the Redmine install in the image if it was possible to set the log level from an environment variable.

[tianon]

I think https://www.redmine.org/projects/redmine/wiki/RedmineInstall#Logging-configuration is accurate/up-to-date on how this is supposed to be configured. I think it would probably be possible to use a snippet that reads that from the environment instead (since it's just raw Ruby code, after all), but unfortunately we consider doing so by default to be out-of-scope in this repository (as doing so in a way that respects custom user configurations is challenging, and our configuration code is already more complex than we're comfortable with).