Consider rename `-dev` image.

I discussed with @yahonda .

`-dev` means HEAD version of `master` or `main` by some people. But `-dev` image in this repository means contain development tools like gcc, gdb and git. 

Should we rename it to `-devtools` or something?

Or rename the current normal image to `master-nighty-slim` and `master-dev-nighty` to `master-nighty`?

