Fix the linux install based on testing. (#104)

## Summary:
It looks like the linux install of khan-dotfiles has bitrotted a bit.
This fixes it up based on a new test regime that occured to me: just
use a docker container for testing!  I wrote up how to do this in the
README.

The main changes I made:
* Add a few packages that might not be installed by default
* Make sure /usr/bin/python is installed
* Make sure the git ssh key is installed correctly

Issue: none

## Test plan:
As described in the README.  After running `make` I could run
`make serve` (in webapp) with success!

Author: csilvers

Reviewers: dbraley, #infra-platform

Required Reviewers:

Approved By: dbraley

Checks:

Pull Request URL: #104

README.markdown

@@ -57,6 +57,24 @@ A few notes to keep in mind:
 Ask in `#infrastructure-devops` if you have any questions, and thanks
 for the contributions!
 
+## Testing `khan-dotfiles` on Linux
+
+It is easy to test the Khan-dotfiles on linux using a Docker container.
+Use a command like `docker run -it ubuntu:22.04` to create a new
+linux instance.  Then run:
+```
+# apt-get update
+# apt-get install -y git sudo make
+# useradd -U -d /home/testuser -m -s /bin/bash -G sudo testuser
+# echo testuser:U6aMy0wojraho | chpasswd -e    # the empty password
+# su - testuser
+$ mkdir -p khan/devtools
+$ cd khan/devtools
+$ git clone https://github.com/Khan/khan-dotfiles
+$ cd khan-dotfiles
+$ make
+```
+
 ## Credits
 
 The `khan-dotfiles` are now maintained by the DevOps group within the

containers/kabuild/README.md

@@ -24,7 +24,7 @@ The only skaffold command we're really using here is "skaffold build"
 
 ## Why NOT linux-setup.sh
 
-linux-setup.sh & linux-functions.sh could possibly be used instead of
+linux-setup.sh could possibly be used instead of
 doing things explicity in the Dockerfile. But that was tried and there was
 quite a bit to debug. And in the long term, DevOps anticipates that all
 services are built with relatively simple containers instead of a mega