I think this should be updated to mention that before running snapcraft from the container, apt update needs to be run, because as it is the snapcore/snapcraft docker image has out of date apt sources and as such snapcraft will fail to install/download packages when using apt because the sources/sites are out of date. So basically the command we should document for folks to run when using this docker image is:
docker run -v "$PWD":/build -w /build snapcore/snapcraft bash -c "apt update && snapcraft"