Add requiements.txt installation to append builder

[rpasricha]

Currently docker and cluster builders pip install the user's requirements.txt file if provided, but not the append builder.

[r2d4]

Here's two solutions:

1. create a fake virtualenv, run pip install within new virtualenv, and package that up.
2. ignore requirements.txt and package up existing virtualenv

[karthikv2k]

create a fake virtualenv, run pip install within new virtualenv, and package that up.

This might be running under OS/env that might work with the base container right?

ignore requirements.txt and package up existing virtualenv

Same as above. Actually, I remember you bringing catching this issue in one of our meetings.

[panbalag]

@r2d4 for the solutions, did you imply to package the virtualenv in to the base image?

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[panbalag]

@r2d4 is there any plans of fixing the append builder code to install packages from requirements.txt?