add support for requirements.txt

**Is your feature request related to a problem? Please describe.**
Make the python dockerfile run `pip install requirements.txt` if the file exists at the root

we could inline it with something like `[ -f "/requirements.txt" ] && pip install requirements.txt` so that it only runs if the file is there

**Mention what platform you want to support the new feature**
oss draft

**Describe the solution you'd like**
when a repo is supplied to draft with a requirements.txt, it should install them

when a repo is supplied without a requirements.txt, this step should be skipped

**Describe alternatives you've considered**
making a new prompt or separated dockerfile for "python_with_requirements"- but this seems like extra complexity and maintenance burden when we could just add it to the one dockerfile we already have

**Additional context**