Remove step to install Bundler in Dockerfile
Remove step to install Bundler in Dockerfile

I think we could have already done so when upgrading Ruby to version
2.6.x (which also included the Bundler gem), but since we didn't, now
that we've upgraded to Bundler 2.x it's probably a good moment.

consul/consul,consul/consul,consul/consul,consul/consul,consul/consul