Dynamically load/create env vars on a per-project & per-machine basis

[ath]

In the devcontainer.json file, I want to mount a local directory, specifically my SSH folder, into the container. This initially seems straightforward:

"mounts": [
  "source=${localEnv:SSH_HOME},target=/root/.ssh,type=bind,consistency=cached"
]

The problem is: I and every other developer needs to set up the $SSH_HOME env var on his machine, which pollutes the global env var space. For 99% of the time my OS is running, this env var isn’t needed. Additionally, I may have multiple VS Code instances running simultaneously, each requiring its own specific env var (with that identical name).

I'm looking for a way to use a placeholder in the source field that would be a valid entry to commit to Git. This could be either executable code (running a local script that computes a return value) or a mechanism to load specific variables (key/value pairs) from a file not checked into Git, which each dev sets up manually on his machine. While executable code is a potential solution, it might be challenging to create something that runs on both Linux and Windows.

Solutions I've already considered:

1. Creating a local bash/PowerShell script that sets up the vars and then starts VS Code. The drawback is that I can't close and open other folders without exiting VS Code and running a new script in the shell.
2. Using initializeCommand doesn't work. I can run a process that creates env vars, but they disappear as soon as the process ends.
3. I created a plugin that sets process.env.SSH_HOME = '...', but this didn't work either. I even marked it in package.json as 'extensionKind': ['ui'] to ensure it runs when opening a devcontainer.

Is there a recommended approach to this issue that allows for flexible, developer-specific configuration without polluting the global environment or requiring constant manual intervention? Or should we consider adding a new feature to devcontainer.json for flexible, developer-specific configurations?

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