Add `--ssh` flag to forward the host's SSH agent socket into the
container, so we can use SSH authentication for things like cloning
private repos, and also updates the socket path every time the container
starts to handle socket path changes like reboot/re-login.
Closes#498
- Closes#150, #394.
- Introduces `AttachmentConfiguration` type so that we can add key-value
options to `--network` in the future.
- Eliminates redundant `ContainersService.Item` type.
- Since we now ensure at ContainersService that hostnames will not
conflict, the network helper IP allocator now simply provides the
existing IP if for an allocation on an existing hostname, which should
handle (in an eventually consistent way) the case where a container
fails to deallocate an IP on shutdown.
J Logan