feat(docker): Honor the host specified in current docker context

[rajiv-k]

One can have multiple docker hosts that they manage and can switch between them using the "docker context" command.
Previously, we used to connect to either the "default docker host" (for example, unix:///var/run/docker.sock in case of Linux/macOS) or to the host specified via the "DOCKER_HOST" environment variable. However, if one switched to a different context, we had no way of detecting that.

Now, we determine the docker host that we connect to by evaluating the following sources in decreasing order of precedence:

• value of "DOCKER_HOST" environment variable
• host specified in the context set via the "DOCKER_CONTEXT" environment variable
• "default docker host" as applicable, otherwise

Fixes: #285

[jesseduffield]

Can you explain what's happening from here to line 380?

[rajiv-k]

Sure! Here's the gist of what's happening.

Line: 364: We create a new Store by passing the path to store directory ($HOME/.docker/contexts) and the "store config" (which is used to return a typed value from a "generic" value that the Store can store).
Here, we are interested in retrieving the information associated with a docker context aka the EndpointMeta, which contains the docker host information. That is why the "store config" includes a getter meant for the EndpointMeta (Line: 359).

The rest of the surrounding lines are just plumbing code to extract the host information and error handling.

Bonus:

What's inside the contexts directory?

$ tree ~/.docker/contexts/meta/
$HOME/.docker/contexts/meta/
├── 2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae
│   └── meta.json
├── ed027fa4e9dc0950c3d3acf4a7bbaf242810ff73290de387e34e6d26c2a5b21b
│   └── meta.json
├── f24fd3749c1368328e2b149bec149cb6795619f244c5b584e844961215dadd16
│   └── meta.json
└── fe9c6bd7a66301f49ca9b6a70b217107cd1284598bfc254700c989b916da791e
    └── meta.json

Those directory names are the sha256 sum of the of the context name. For example, one of my context is named colima .

$ echo -n "colima"| openssl sha256
(stdin)= f24fd3749c1368328e2b149bec149cb6795619f244c5b584e844961215dadd16

What's inside the meta.json?

$ cat $HOME/.docker/contexts/meta/f24fd3749c1368328e2b149bec149cb6795619f244c5b584e844961215dadd16/meta.json | jq '.'
{
  "Name": "colima",
  "Metadata": {
    "Description": "colima"
  },
  "Endpoints": {
    "docker": {
      "Host": "unix:///$HOME/.colima/default/docker.sock",
      "SkipTLSVerify": false
    }
  }
}

[lclpedro]

Is not resolved if you create link to colima with default docker.sock?
sudo ln -f -s ~/.colima/default/docker.sock /var/run/docker.sock

[rajiv-k]

@lclpedro Creating the symlink would work. But the issue is that if you manage multiple docker environments, you will have to re-create these symlinks whenever you want to target a different environment (as described in linked issue #285). docker context exists to solve this very problem.

[jesseduffield]

Sorry this took me so long to get back to @rajiv-k . Great work!

[whobutsb]

Thank you for adding this feature! I added the feature request in December 2021. Really happy to see it get added.

[whobutsb]

@rajiv-k - I was testing out this new functionality. Is honoring the docker context only available for http connections? Am I able to use ssh connections? When I changed my docker context and ran lazydocker I wasn't able to connect. Here is the inspection on my docker context:

> docker context inspect uj-staging
[
    {
        "Name": "uj-staging",
        "Metadata": {},
        "Endpoints": {
            "docker": {
                "Host": "ssh://uj-staging",
                "SkipTLSVerify": false
            }
        },
    }
]

Error Message:

[rajiv-k]

@whobutsb Looks like a possible regression. I am working on a PR to make this work (again).
Could you please create a new issue for it?

Update: This commit fixes the above issue with remote docker daemon over ssh.