Support Coder-specific Devcontainer customizations

[johnstcn]

Problem

In order to allow the full coder_app experience with applications running inside a devcontainer, we need to allow decoupling coder_apps from the template. Currently a coder_app is only defined inside the template.

Proposed Solution

Allow devcontainers to declare a subset of the coder_app spec inside customizations.coder. This can then be read by the agent and used to dynamically create / update coder_apps for the workspace agent.

For example:

{
  [...]
  "customizations": {
    "coder": { // or maybe `com.coder.coder` instead?
      "apps": [
        { // we can basically allow the entire coder_app spec in here, within reason
          "slug": "code-server",
          "display_name": "Code Server inside your Devcontainer!",
          "url": 13337,
          "icon": "/icon/code.svg",
          "share": "owner",
          "healthcheck": { [.. ] }
        }
      ]
    }
  }
}

[johnstcn]

As far as I can tell, the entire coder_app spec will be useful here (except for agent_id; this will be implicit from the CODER_AGENT_TOKEN).
https://registry.terraform.io/providers/coder/coder/latest/docs/resources/app

[defelmnq]

After internal discussion and working on a PoC - here's a status update :

With the current project status, achieving this ticket would :

• require an update on the AgentAPI to create new calls and send the applications defined in the devcontainers.json file.
• require to insert these apps in DB after the devcontainer up step.

In the meantime, we are working on devcontainers agent that will give us a better visibility and :

• let us rely on agent lifecycle to refresh apps
• know about which port are being used internally for each app
• have different AgentAPI calls - with different payload

As we know this one will not be part of the May release, but also the devcontainers agent RFC is moving well - does it make sense to continue to focus on a first version that will at best have to be erased, at worst be changed even before being released ? Considering also the pain to update the AgentAPI and the compatibility we have to keep.

cc @mtojek