* add polkadot build script

* Add scripting to bring up a simple alice-bob example net

Demonstrated to produce blocks, but as of right now there's still
trouble getting it to respond to external queries on its ports.

* enable external rpc access to the nodes

Also shrink the build context by excluding some extraneous data.

* Ensure external RPC access works

Also set default branch appropriately, and have the stop command
clean itself up more thoroughly.

* Add multi-stage dockerfile for building the cumulus-test-parachain-collator

- Exclude the docker/ directory from build context because we're
  never going to build recursively, and this prevents spurious
  cache misses
- build the parachain collator in three stages. The build stage
  is discarded; the collator stage has a wrapper script to simplify
  generating the right bootnodes flags, and the default stage
  has just the binary in a small runtime.
- build_collator.sh collects appropriate...