Worker containers for applications

What problem will this feature address?

I like to add "workers" to an application that handle load, e.g., when following a command bus pattern.

Workers use the same codebase / build but have a different a different start command (eg., not running web server, but a script that performs actions).

Currently you cannot use the same build for different applications. Deploying app + worker woudl require building the code twice. Thats an overhead.

Workers stop after X minutes to prevent memory flooding. Docker Swarm can handle the recovery.

Describe the solution you'd like

I like this interface from Upsun:

Workers should be able to be "attached" to an application. They inherit the build and volumes. You should be able to override env vars for instance and docker swarm settings. They dont need domain attachments.

Describe alternatives you've considered

As a workaorund, you can create a new application (using the same codebase) but different start command (e.g. RAILPACK_START_CMD).

Additional context

No response

Will you send a PR to implement it?

No

The issue names no files, tests, or entry points. Begin by reviewing how applications, builds, volumes, start commands, environment variables, and Docker Swarm settings are currently represented; done would support attached workers that inherit the build and volumes while allowing overrides and independent start commands.