add RAILS_ENV to prod & staging Dockerfile (#3332)
add RAILS_ENV to prod & staging Dockerfile (#3332)

* add docker RAILS_ENV arg & env

As the asset pipeline writes to a non-default location (https://github.com/zooniverse/panoptes/pull/3249)
ensure the RAILS_ENV is set at build time to correctly wire up the assets in the expected dir

* ensure the secret key is a string

fixed in rails 5+ https://github.com/rails/rails/commit/7a9ce69ce081a0c186a23806c1af1f71c697ea77
zooniverse/Panoptes,camallen/Panoptes,camallen/Panoptes,camallen/Panoptes,zooniverse/Panoptes,zooniverse/Panoptes,zooniverse/Panoptes,camallen/Panoptes