README.md: repo renamed to clearify purpose

This Docker will configure to *use* a proxy, not *be* a proxy.

Signed-off-by: Paul Campbell <paulcampbell@fife.ac.uk>
etc/environment: set *_proxy environment variables

Are these even used when a docker process runs?

Signed-off-by: Paul Campbell <paulcampbell@fife.ac.uk>
Dockerfile: added

Signed-off-by: Paul Campbell <paulcampbell@fife.ac.uk>
etc/apt/apt.conf.d/60proxy: configure apt to use proxy

Signed-off-by: Paul Campbell <paulcampbell@fife.ac.uk>