Ability to specify rti image for dockerized federated programs

[lhstrh]

• Add rti-image attribute and identify locally-build rti as rti:local
• Update all federated tests that use docker to use rti:local
• Update docs (Rewrite of Docker docs lf-lang.github.io#180)

Note that the FROM attribute should be lowercase, but this won't work until #2136 is merged due to a collision with the from keyword.