`containers` field in `NetworkDetails` can be null (on Docker Swarm)

[pitkley]

Calling Docker::new().networks().list(&Default::default()).await on a Docker Swarm manager node (potentially also on non-manager nodes, I wasn't able to reproduce this though) causes the following error:

SerdeJsonError(
    Error("invalid type: null, expected a map", line: 1, column: 364),
),

I have analyzed this and have identified that in a Swarm Docker will have a special network called ingress which, as far as I understood, is used for routing ingress traffic within the cluster to a worker node that can respond to it. Apparently this network can never have containers associated to it, which the Docker API under GET /networks (as called by .networks().list()) seems to indicate by returning a null in the containers key instead of an empty object. For clarification, following is the reduced output for GET /networks on a Docker Swarm manager node:

[
  {
    "Name": "ingress",
    // ...
    "Containers": null,
    // ...
  },
  // ...
  {
    "Name": "bridge",
    // ...
    "Containers": {},
    // ...
  },
  // ...
]

Unfortunately shiplift expects this field to always be present (@v0.7.0, @master), which causes the error shown above.

A simple non-breaking solution would probably be to just annotate the field with #[serde(default)], for which I'm happy to open a PR, but I'm noticing that since 0.7.0 some major refactorings seem to have taken place, where backwards-compatibility might not be a concern and actually redefining the field as Optional<HashMap<...>> might be the "more correct" solution.

Do you have any preference on the solution chosen? I'm happy to provide a PR for either one.

---

• Crate version: 0.7.0, master @ 9750d29
• OS: Linux (CentOS 7)
• Output of running docker version on the command line:

Client: Docker Engine - Community
 Version:           20.10.6
 API version:       1.41
 Go version:        go1.13.15
 Git commit:        370c289
 Built:             Fri Apr  9 22:45:33 2021
 OS/Arch:           linux/amd64
 Context:           default
 Experimental:      true

Server: Docker Engine - Community
 Engine:
  Version:          20.10.6
  API version:      1.41 (minimum version 1.12)
  Go version:       go1.13.15
  Git commit:       8728dd2
  Built:            Fri Apr  9 22:43:57 2021
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          1.4.4
  GitCommit:        05f951a3781f4f2c1911b05e61c160e9c30eaa8e
 runc:
  Version:          1.0.0-rc93
  GitCommit:        12644e614e25b05da6fd08a38ffa0cfe1903fdec
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0