Docker images: Document supported entrypoints

[RyanGlScott]

The crux-llvm and crux-mir Docker images currently use the crux-llvm command and the cargo crux-test command as their respective entrypoints. However, these are not the only ways of using the Docker images:

• In both images, one can use a shell (e.g., bash) to use the images interactively.
• For the crux-mir image, one can use other commands such as crux-mir, cargo saw-build, crux-rustc, saw-rustc, etc.

We should document all of the alternative entrypoints that we support, along with examples of how to invoke them, what their command-line options are, and other relevant information.