git user/uid issue when building in docker

[gonuke]

The current instructions offer a path that uses a docker image to build the website. To avoid polluting the docker-mounted filespace with files owned by root, the docker image is started with the UID of the current user.

The first step in a few (related) targets is to run git fetch once the docker image is launched, but the UID of the current user doesn't exist in the docker image, so git throws an error.

Unless we can fix this inside the docker image (probably hard in general) we probably need to ensure that fetch happens prior to launching the docker image.