weave env --restore

[rade]

weave env stashes the existing DOCKER_HOST in ORIG_DOCKER_HOST, and weave env --restore sets DOCKER_HOST to $ORIG_DOCKER_HOST.

Note that weave env --restore always executes locally, which means it will work when the env has a 'broken' DOCKER_HOST.

Closes #1288.

[rade]

@bboreham would appreciate some feedback here on what this does and how it does it.

Possible point of contention...

• weave env --restore does not unset ORIG_DOCKER_HOST, so the env is not the same as before weave env was invoked. However, that means weave env --restore is idempotent, which imo is desirable

[rade]

When weave env --restore is invoked without a DOCKER_HOST_ORIG, weave prints a warning on stderr and exits with 1. Alternatives I've considered: a) silent exit 0, b) silent exit 1, c) warning and exit 0. Anybody want to argue for a different choice than the one I made?

[rade]

@bboreham I think this is ready modulo docs and squashing. Would appreciate some feedback first.

[bboreham]

The implementation lgtm.

I am unclear how prevalent the set-up is where setting DOCKER_HOST in your regular environment just works. It doesn't do anything for me because the sockets are protected so I have to sudo every command.

[rade]

The new command also works in situations where DOCKER_HOST starts off unset/null. i.e. weave env --restore sets it to back to blank. (NB: minor wart: it is not unsetting DOCKER_HOST, only setting it to null).

[rade]

Umm. This wasn't ready to merge. Still needs docs!