Allow providing response file for running non-interactively

[hilverd]

This allows users to provide a config/response.txt file with the commands to be used for connecting and logging in. If that file is not present, start-vpn should work as before.

I had to apt-install gettext-base so that the envsubst command is available for replacing the string $VPN_PASSWORD in the response file with the value of that environment variable (which is set after prompting the user for their password).

If the Docker image already exists, start-vpn will now still make a call to docker build which should be quick if nothing has changed (and I've added -q as it seemed appropriate). This is because existing users will need to rebuild their image to get the changes to Dockerfile and entrypoint.sh.

I also added a small bug fix

chmod a+r /etc/resolv.conf

to fix a problem on my machine caused by config/resolv.template not being world-readable.

[danjpgriffin]

Thanks Hilverd