Add script and dockerfile to build an injected docker image for the polkadot-collator binary

[chevdor]

This PR adds a new script and a dockerfile used by this script.

The docker image that is built contains the polkadot-collator image and also copies the chain specs in the image under /specs. That allows users to run the parity chains without having to mount folders from the repo while users of other chains can still mount their specs in the container under /specs.

The image can be called for instance as:

OWNER=parity
IMAGE_NAME=polkadot-collator

docker run --rm -it $OWNER/$IMAGE_NAME \
   --collator --tmp \ 
   --parachain-id 1000 \ 
   --execution wasm \
   --chain /specs/westmint.json