Docker base image with PostgreSQL 10 released

[tgxworld (Alan Tan)]

@ixeft

but maybe we shouldn’t have changed the locale (in this case, it might not be useful to put in the sample app.yml this env variable)

Hmm that is a good point actually. @sam Do you know if Discourse search would be affected if the LANG is configured differently? Otherwise, I think we should just bake the ENV into postgres.template.yml.