Fix random errors when building RoadRunner docker image

[acelaya]

Currently, the rr binary is downloaded via vendor/bin/rr get, which donloads it using GitHub's API. This API has some limits when not providing an auth token (which is the case when building a shippable docker image).

One possible solution would be to copy the binary from the official RoadRunner docker image, as documented here: https://roadrunner.dev/docs/intro-install#docker

The main problem is that this logic needs to be conditional based on the SHLINK_RUNTIME env var.

[acelaya]

See:

• https://docs.docker.com/engine/reference/builder/#onbuild
• https://forums.docker.com/t/dockerfile-use-dynamic-image-name/133335