How does the Docker plugin get the container to push?

I have my pipeline set up to build the source code from my github repo, which looks something like this:

    image: ubuntu/16.04
      - apt-get update -yq
      - apt-get install -yq autoconf automake build-essential
      - make build
      - make test

When that step completes I want to package what I have built into a docker container and push it to docker (or, in my case, GCR). I set up the GCR plugin but it appears to try to rebuild everything using a Dockerfile in my github repo. Is there some way to easily package what got built in the build stage into a docker container? Or do I need to use a bunch of docker and gcloud commands to do it manually, building the docker image within drone? I was hoping to do the one build in the beginning, then use it to create a few different docker containers that all had different entrypoint cmds (like one for the client version of my app, and one for the server, etc.)