Change default ports when Thruster is present

[luizkowalski]

Over the weekend, I realized that the exposed port dockerfile-rails generated when Thruster is present is insufficient to make Thurster work transparently. I summed up my findings here but long story short:

• Thruster's default port is 80
• Thruster's default Puma port is 3000
• dockerfile-rails exposes 3000
• Services like Traefik, which checks for EXPOSE directives to route traffic to the correct port, will bypass Thruster completely in these cases

I think the easiest solution is to keep exposing 3000 for the sake of backward compatibility (and everyone expects Rails to expose 3000 anyway) but bind Thruster to 3000 and Puma to e.g. 3001, something like this:

if thurster?
 ARG TARGET_PORT=3001
 ARG HTTP_PORT=3000
 
 EXPOSE ${HTTP_PORT}
end

I can take a stab at a PR for it unless you have a better idea

[luizkowalski]

hmm, Rails will expose 80 apparently, so that's probably easier...