Empty ${localEnv:} in mount source should skip the mount, not pass empty source to Docker

[NickBorgers]

Problem

When a devcontainer.json defines a mount using ${localEnv:SSH_AUTH_SOCK} and the environment variable is unset or empty, the CLI resolves it to an empty string and passes source=,target=/run/ssh-agent.sock,type=bind to docker run. Docker rejects this:

invalid argument "source=,target=/run/ssh-agent.sock,type=bind" for "--mount" flag: invalid value for 'source': value is empty

This is particularly problematic when using pre-built images (via devcontainers/ci) because the mount is baked into the image's devcontainer.metadata label and cannot be overridden at runtime (the spec merges mount arrays by concatenation).

Expected behavior

When ${localEnv:VAR} resolves to empty in a mount's source, the CLI should skip that mount entirely rather than passing an invalid empty source to Docker.

Reproduction

1. Create a devcontainer.json with:

   "mounts": [
     "source=${localEnv:SSH_AUTH_SOCK},target=/run/ssh-agent.sock,type=bind"
   ]

2. Run devcontainer up in an environment where SSH_AUTH_SOCK is unset
3. Observe Docker failure due to empty mount source

Context

• CLI version: 0.85.0
• Related: Request: option to disable mounts.  ci#166 (request to disable mounts in CI builds)
• This affects any CI/CD environment that uses pre-built devcontainer images where the build host had SSH_AUTH_SOCK set but the runtime host does not