Tilt, docker build, and ssh-agent forwarding

gistfile1.txt

This took a while to figure out because the information is a bit spreadout and I was missing a piece of the puzzle.

SSH agent forwarding:
Set up ssh-agent for github and ensure your local key works 
(see https://docs.github.com/en/developers/overview/using-ssh-agent-forwarding)

Tiltfile
Add ssh="default" as a parameter to the docker_build which which needs to pull from github 

docker_build( 'localhost:32000/something', '../something',
    dockerfile="./dockerfiles/Dockerfile.dev",
    build_args={
       "RUBY_VERSION": "2.7-alpine",
       "NODE_ENV": "development",
       "RAILS_ENV": "development",
       "BUNDLER_VERSION": "2.2.25"
    },
+   ssh="default",
    live_update=[sync('../something/','/app')],
  )

Dockerfile
Add the following at the appropriate places in your dockerfile.

RUN apk add --no-cache openssh-client 
RUN mkdir -p -m 0600 ~/.ssh && ssh-keyscan github.com >> ~/.ssh/known_hosts
RUN --mount=type=ssh yarn install

You'll need to install a ssh client if there isn't one present, add github as a known ssh host, and finally any command that needs access will need the --mount=type=ssh.

You can similarly run `RUN --mount=type=ssh ssh -T git@github.com` to confirm that your ssh credentials are forwarded.