Update 01b-docker.md

[rob-c]

Just running through this to setup a docker container to avoid throwing up a full VM to do some work.

I hit a problem, externally to CERN, the URL (command) wrapped in the PR will hang due to the firewall.

This PR is just adding a URL that works externally from CERN so that external users don't have to do proxy-ing.

Not sure if there is a policy on this but new users won't be overly familiar with why things like this don't work from their home institute or on their laptop.

[chrisburr]

@rob-c I always use SSH to access gitlab even when away from CERN so I guess this is due to your institute's firewall rather than the CERN one?

Maybe putting this would be clearer:

This command will fail from some institutes due to their firewall. If this happens you can instead clone using https:

[rob-c]

@chrisburr Potentially, I'm traveling on eduroam (and use that in my home institute), and typically find I don't have a problem with the https way of connecting.

Would it be a better idea to re-phrase it as, try this and drop back to https if this fails?

[chrisburr]

@rob-c Yeah I think that would be good idea. A notable minority of places block non HTTP(S) traffic (i.e. port 80 and 443).