Be able to specify customized user instead of hardcoded "dev"

[shiywang]

Once VScode support this microsoft/vscode-remote-release#46
we can use buildArgs to specify our own user like USER=$(id -un) instead of dev.

[vfreex]

If you are running dev container with docker on Linux, you just need to make sure that the UID in the container matches your actual UID, while the value of username doesn't matter.

But of cause it would be nice to make the username matches the actual username to avoid confusion.

[vfreex]

#104 switched to use docker-compose.yaml to build images. We can add a build arg there.

[sosiouxme]

This happened at some point :)