Accept tcp protocol for DOCKER_URL

[kinghuang]

Docker API endpoints are typically specified using tcp as the protocol rather than http or https. It would be nice if promswarmconnect accepted tcp in the DOCKER_URL value.

Right now, promswarmconnect throws an error if tcp is specified as the protocol.

Get tcp://docker-host-here:2376/v1.24/tasks?filters=%7B%22desired-state%22%3A%5B%22running%22%5D%7D: unsupported protocol scheme "tcp"

[joonas-fi]

You're right. It is weird the way Docker does it, since http | https is what goes on top of TCP (and TCP doesn't tell anything about the application-level protocol), but you're correct that we should use the same notation that Docker does it because it's confusing if promswarmconnect does it in a different way.

I need to investigate a bit more, but right now it seems that we'd need to behave this way:

• tcp:// => http
• tcp+tls:// => https

[kinghuang]

The Docker CLI and related tools always use tcp://. And then, the DOCKER_TLS environment variable is used to specify if TLS is on or not, and DOCKER_TLS_VERIFY for whether the cert is validated. They also use DOCKER_HOST to specify the Docker endpoint.

https://docs.docker.com/engine/reference/commandline/cli/#environment-variables

Ideally, I think DOCKER_URL should map tcp to http, as you've written. But, map tcp to https if DOCKER_TLS is set.