* Add docker-dhcp-relay/Dockerfile to .gitignore
* Add isc-dhcp-relay .deb package to image build process, along with my Option 82 patch
* Install custom isc-dhcp-relay in dhcp_relay docker
* Install isc-dhcp-relay build dependencies in sonic-slave Docker container
* Copy the built .deb package to the destination directory
* Add dependencies for isc-dhcp-relay
* Change Option 82 string to '<hostname>:<portname>'
* Install dependencies of .deb files implicitly in Dockerfile
* Remove unused line
* Remove unnecessary space
