Describe the feature

Some projects need external dependencies installed, such as ffmpeg, youtube-dl, or things related to cmake.

This feature allows users to specify dependencies in Shuttle.toml (or macros?) that are then installed to the shuttle container before build time using apt or other suitable way (nix??). This allows more and more "niche" projects to deploy.

Or perhaps just allow an arbitrary shell script to execute before the build stage?

Suggestion/example of how the feature would be used

Shuttle.toml

dependencies = [ "ffmpeg" ]

or

apt = [ "cowsay" ]
nix = [ "libopus" ]

or arbitrary script, say shuttlebuild.sh

apt install -y cmake
rm -rf / # 🤪jk

Not sure which approach suits shuttle the best.

EDIT: Realized this is a duplicate of #135
EDIT 2: Added script example