Consider deprecating docker.host, docker.context, etc. settings

[bwateratmsft]

We're thinking about deprecating these settings, in favor of using VSCode's standard terminal.env.<os> option. I think that we would want to have a migration experience to help users get over to the new settings, as well as some documentation to describe how to set those.

VSCode will apply those environment variables automatically to all terminal windows, but we would need to pick up and apply the appropriate setting for any shell tasks that we execute behind-the-scenes.

[bwateratmsft]

/cc @chrmarti

[bwateratmsft]

From telemetry data:

Config Source	Percentage
Default context	97.3%
<unknown>	1.1%
DOCKER_HOST env var	0.6%
Custom context	0.6%
docker.host setting	0.3%
docker.context setting	~0%
DOCKER_CONTEXT env var	~0%

So the usage of these settings is extremely low.

[bwateratmsft]

@Tyriar had an alternative idea I think I like better. Instead of using terminal.integrated.env.<os>, define our own setting that accepts any key/value pairs, e.g.:

"containers.envVariables": {
    "DOCKER_HOST": "...",
    "SOME_VAR": "SOME_VALUE"
}

We can apply this setting to ExtensionContext.environmentVariableCollection, as well as any shells we create in the background.

[chrmarti]

I assume that could contain env variables for several CLIs? (E.g., Docker and Podman.)

[bwateratmsft]

Yes, I think we would make it accept any environment variables, so that we don't have to chase many runtimes' particular environment variables.

[bwateratmsft]

Associated docs PR: microsoft/vscode-docs#5488

[bwateratmsft]

This has now been released in Docker extension version 1.23.0.