Add Doxygen to Base Image?

[stephendgilbert]

Right now my students have to manually install doxygen each time we use it (it doesn't persist in the image). It's fast to install and takes only a little memory, but it's still kind of a hassle. Right now in my classes I use your .devcontainer.json file as part of the student's codespace repo, so it seems to be built from the CS50 image. Reading the docs, I haven't found a way to modify that to actually add doxygen. I think it needs to be done at the DockerFile stage.