Adds BACKEND_API and ELECTRS_API_URL environment variables which will be populated into backend/mempool-config.json on startup. This will allow docker users to easily switch to electrs mode and specify a server if they have a need to do so.
HTTP_PORT was ignored and can be remapped by Docker anyway. Remove jq as no longer needed by entrypoint.sh Added bash as needed by entrypoint.sh
Add files for the creation of docker container
