Support passing environment variables in to agent containers

[bnavetta]

This adds a mechanism for passing environment variables in to self-hosted agents. The intent is that this is used for:

• Host-specific configuration (e.g. pointing tools at a specific mounted volume)
• Injecting secrets which are not managed by Warp

The configuration syntax follows the Docker CLI:

• --env KEY=value to set an environment variable directly
• --env KEY to inherit a value from the host environment
• -e as a shorthand

Testing

I ran a self-hosted worker locally with the --env flag and asked an agent to print all environment variables.