Support configuring the environments omitted from bundle install

[DevinFrenze]

because of ruby-generate-dockerfile/app/Dockerfile.erb lines 159 and 161, ruby projects (in my case in the Google App Engine flexible environment) may not deploy using environment names test or development.

although there's a convention to use development locally, I like to use local as the name of my local environment and tried to use development as the name of a deployed environment. however I was stuck with a 502: Bad Gateway status because my dependencies aren't installed when the env is development.

i think this docker file either shouldn't make assumptions about environment names, or should make those assumptions known and allow for some kind of configuration (on App Engine it could be some optional settings in app.yaml)

[dazuma]

Because the convention is to use development as the local environment, I think the current default behavior remains correct for most users. But I can see an argument for making it configurable via app.yaml.