output of Docker-based build process results in files owned by root

When using the Docker container to build SSM binaries, all of the files it produces as its final output are owned by `root` after the build process is completed and the container has exited. This is true even when running the container from a non-root user.

While this probably isn't a major problem for most people (because they're probably going to install the binaries as root anyway), it is a bit of an inconvenience for the minority of us who only want to keep it in a self-contained environment in a home directory of a non-root user (instead of installing into a system area).

I have a pull-request coming soon, encapsulating changes to address this issue. I just needed to create this issue first, so that I could use its ID as part of the name of the branch.