bibtool not available in provided dev container

[StevenClontz]

I use GitHub Codespaces to contribute to mathlib, and realized today that at

mathlib4/docs/references.bib

Line 3 in 965c7f8

# bibtool --preserve.key.case=on --preserve.keys=on --pass.comments=on --print.use.tab=off -s -i docs/references.bib -o docs/references.bib

we ask contributors to run bibtool. However, bibtool is not available on this provided Codespace.

I was able to install it successfully with sudo apt update and sudo apt install bibtool, but ideally it would be provided as part of our Docker image or installed with a postCreateCommand automatically.