Support ${env:-} in the dockerRun/volumes/localPath

[psaxton]

Issue: Currently, the only variable allowed for substitution in the dockerRun/Volumes/localPath value is ${workspaceFolder}. When trying to use another value such as ${env:HOME}, an error occurs on build:

Command failed: docker run -dt -P --name "MyProject" -v "${env:HOME}/.aws/:/root/.aws:ro"

/bin/sh: 1: Bad substitution

Example:

   "dockerRun": {
      "volumes": [
         {
            "localPath": "${env:HOME}",
            "containerPath": "/root/.aws",
            "permissions": "ro"
         }
      ]
   }

Use case: When developing applications accessing AWS services, configuration and, more importantly authentication credentials, are stored in ~/.aws. Docker requires absolute paths for volume values. The home directory is user dependent.

Plugin version: Problem encountered with version 0.6.4

[psaxton]

This issue is related to but not solved by #785.

[philliphoff]

@psaxton This is a good idea, but likely requires custom logic to parse and resolve environment variables. (I don't believe VS Code provides an API for such resolution, which is why I've only supported such substitutions on an as-needed basis.)

[bwateratmsft]

Fixed in #1245 since we don't inspect the volumes, just pass along to ShellExecution.