Update docker build instructions

[sluzynsk]

The docker build instructions currently state to run:
docker run --volume=$(pwd):/home/circleci/project olback/rust-gtk-linux cargo build --release

That docker container is out of date and listed as deprecated on dockerhub besides. The app builds correctly with the new container:
docker run --volume=$(pwd):/home/circleci/project olback/rust-linux-gtk cargo build --release