Allow the GH bridge to run without a server (and use a local Git directly)

[samoht]

For instance, you could do:

$ datakit-bridge-github -r samoht/test -d git:///tmp/foo
Starting datakit-bridge-github %%VERSION%% (*:rw)...
Connecting to git:///tmp/foo [github-metadata].

And then:

$ tree /tmp/foo
.
└── samoht
    └── test
        ├── pr
        │   └── 32
        │       ├── base
        │       ├── head
        │       ├── state
        │       └── title
        └── ref
            ├── heads
            │   ├── adassad
            │   │   └── head
            │   ├── bar
            │   │   └── head
            │   ├── master
            │   │   └── head
            │   ├── samoht-patch-1
            │   │   └── head
            │   ├── test
            │   │   └── head
            │   ├── test2
            │   │   └── head
            │   ├── toto
            │   │   └── head
            │   └── yyy
            │       └── head
            └── tags
                └── titi
                    └── head

16 directories, 13 files

You can then edit the Git repo, and when you commit changes they are reflected directly to GitHub.

[samoht]

The last failure is due to https://github.com/moby/datakit/blob/master/ci/src/cI_docker.ml#L125

From @talex5 on slack:

Ah, maybe we need a third case:

• No FROM override => pull
• Locally-build from => don't pull
• Published image => pull

[samoht]

I am not super fond on how to specify the type of the endpoint, but let's change that in a subsequent PR if that's needed.