Support external dependencies (and emacs-vterm)

[vale981]

This modifies melpazoid.py to generate an extra file with native dependencies that are installed with apt-get when the docker image is built.

The motivation for this is to support emacs-vterm and it's natively compiled module. Maybe I'm overlooking something and this is overkill :).

[riscy]

Thank you, by the way, very much for this. I have been hoping to get something like this sorted out. I'll look at this soon!

[vale981]

any updates?

[riscy]

I apologize for it having taken me so long to get to this. This looks great to me - thank you very much! - although I may make a change or two to increase the amount of layer reuse when building the package.

[vale981]

thanks mate :)