Add script and dockerfile to build an injected docker image for the polkadot-collator binary (#591)

* remove exec flag on json file
* fix dockerignore filter to allow building the injected docker image
* Update docker/injected.Dockerfile
* Update docker/scripts/build-injected-image.sh

Co-authored-by: Martin Pugh <[email protected]>
Co-authored-by: Denis Pisarev <[email protected]>