Add libpq deps to Dockerfile

[grdryn]

Before these changes:

• the container image build fails because it's missing libpq-dev to
  compile against
• the container fails to start because it can't open libpq.so.5

After these changes:

• I think I'm still missing some required config so it still won't
  start successfully, but...getting closer :)

[mattl]

This is great. I'll leave this open until you get the rest working.

[grdryn]

@mattl the image seems to work now, if I run it with something like the following:

podman run --rm -ti -e SECRET_KEY_BASE=1 -e RAILS_ENV="development" -p 3000:3000 hacienda:latest

The -e RAILS_ENV="development" is to work around the fact that it's production by default, but there's no production database available by default. I don't know enough ruby/rails things yet, so I just changed the environment variable to progress.

In my browser at the root page, I see the default rails page. /users shows an error, seemingly because I haven't set up the database yet. Do you have any instructions for that?

[mattl]

None of this is terribly usable yet. But this is useful for getting it work under Linux.