Add support to copy extra files from the Docker image to the package

[jdufresne]

This allows native OS libraries (*.so files) to be copied to the
serverless archive to be available at runtime. This introduces the new
option dockerExtraFiles, a list of paths on the Docker image to copy.

For example, the mysqlclient package requires
libmysqlclient.so.1020. One can now add an item to the
dockerExtraFiles option in their serverless.yml:

pythonRequirements:
  dockerExtraFiles:
    - /usr/lib64/mysql57/libmysqlclient.so.1020

This file will be available at runtime in the Lambda function.

This removes the need to manage these files in a more manual way and
avoids the need to commit binary files one's repository.

[jdufresne]

Rebased to resolve conflicts.

[dschep]

Can you update the docs some more and make sure the diff is actually what's intended?

[jdufresne]

Can you update the docs some more and make sure the diff is actually what's intended?

I'm happy to, is there something specific you're looking for beyond the inline comments? If you provide some guidance towards what you'd like to see, I'll fill in the rest.

[dschep]

ah, i overlooked the "if you used the dockerfile from the prev sect' bit

[crzypatchwork]

I'm having troubles with it. I need to have this OS library for a pip package, but I can't make it work no matter what.

custom:
  pythonRequirements:
    dockerizePip: True
    dockerExtraFiles:
    - ./libsodium.so

raise ValueError('Unable to find libsodium'), in <module>th_frames_removed