Put the whole simulator in the Docker image

[sampsyo]

This changes the Dockerfile to contain a pre-built simulator, which should make it possible to get going without building anything (and without volumes or permission troubles). A couple of caveats:

• I had to move the Dockerfile to the repo root to make COPY work to insert the files into the container.
• I removed the enter_docker.sh script because it's so simple now: just docker run -it ghcr.io/cucapra/gem5-mesh:latest.
• I am pushing a new version of the image to GHCR now (I hope I did that right), which should mean (when it's all done in a couple of minutes) that docker pull ghcr.io/cucapra/gem5-mesh:latest should be all that's necessary.