Allow home directory additions in web container, fixes #926, fixes #1459

[rfay]

The Problem/Issue/Bug:

#926 requests a way to configure the user inside the web container (.bashrc, etc)
#1459 specifically requests a way to set the user's ~/.gitconfig as they apparently do commits inside the container

How this PR Solves The Problem:

Copy the contents of the project's .ddev/homeadditions into the containers ~/

Manual Testing Instructions:

mkdir -p  .ddev/homeadditions
touch .ddev/homeadditions/ITWORKED.txt
ddev start
ddev ssh
ls -al ~/

You should see your ITWORKED.txt

Try out some of the example usages in docs

Review docs.

Automated Testing Overview:

Hopefully existing tests would catch regressions.

Related Issue Link(s):

Release/Deployment notes:

[rfay]

Current artifacts are at https://circleci.com/gh/drud/ddev/16195#artifacts/containers/0 - Would really love to have people try this out. It's a very simple approach, but should help the people that have requested these solutions. @unn @andrewfrench

[andrewfrench]

I'm able to create files in .ddev/homeadditions/ and have them appear at ~/ in the container, as shown with both the IT_WORKS.txt file as well as a .bash_aliases file that configures the container bash environment. This is pretty cool, and while I don't personally work from in the container, should really help those that do.

[unn]

👍 Testing steps followed and worked as expected.