Adds a Dockerfile under the docker to package the corso binary.
This currently requires the user to build the corso binary (for linux/amd64) in the build image
prior to building the docker image. Follow up PRs will introduce a Makefile.
Also moves the build image to the build folder and adds a couple of README.md files with
instructions.
Fixes#218
* New container will have deps known at the time the container was made
* If deps change a lot, rebuild/save current container image as deps
are outside the directory mount
* Deps are not explicitly listed in the container Dockerfile or script
as go mod is expected to manage them for us
* Automatically creates a container user with UID/GID of the current
user to avoid file ownership issues