Get deployed_version from service's Docker container's image

[lehnerpat]

Hello!

Thanks for this great project! I recently discovered Argus and am trying it out as an addition to managing my homelab. So far, I'm really liking it! (In particular, I love how little RAM Argus uses! 🙌)

While adding some of my self-hosted services to Argus, I've found that not all self-hosted apps expose their version on an unauthenticated route (and some don't expose it at all*).
For example, Paperless-ngx only exposes its version on authenticated routes, but doesn't currently have a fine-grained API token mechanism (in fact, it currently only has a single API token per user). So for this case I'd prefer not to put that API token into my Argus config.

However, in my case, most of my self-hosted services run as Docker containers on the same host as Argus. So another way to get the deployed version would be checking which image version each service's Docker container is using.
Theoretically, I could mount the Docker socket into Argus, and then Argus could query it directly (although it would either need to run as root, or I'd have to set up a specific user in the docker group that's allowed to use the socket). IIUC, right now Argus can't send HTTP requests to a Unix socket, and doesn't support running shell commands for checking the deployed version.

So I'd also like to ask for other ideas of how to connect these two dots, i.e. how to expose the image versions of relevant Docker containers to Argus. (I'm thinking of hacking together a small container that reads the image versions of all Docker containers and provides them via HTTP for Argus to query, but if there's an existing solution, I'd prefer that for sure.)

Thanks!

Somewhat related: #541

_{*) Sidenote: One example of a "service" that doesn't expose its version via a web request is the Tailscale VPN docker container, since it doesn't offer any web UI or API itself. This is arguably not a self-hosted web-app, so not sure if this is out of scope for Argus -- but I'd still like to monitor it for updates!}