Ability to set custom path to Docker socket

[9p4]

Clear and concise description of the problem

Set an environment variable to have the Docker client connect to a specific socket. This would enable people to use Podman as their runner (using the Docker compatibility socket). Currently, I have /var/run/docker.socket symlinked to the user's Podman socket /run/user/1000/podman.sock, but this is an inelegant solution at best.

Suggested solution

Add an environment variable (maybe WOODPECKER_DOCKER_SOCKET) that, if set, can set the addr value in the Docker client (https://github.com/moby/moby/blob/88bd93fa603870897400b7d1f0c836d554caf85e/client/client.go#L73)

Alternative

A dedicated Podman runner, but that would be much more difficult to use than using the Podman compatibility layer.

Additional context

#85

Validations

• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[mscherer]

From a quick look at the code, you can set DOCKER_HOST and the underlying client library will do the right thing.

The only issue is that https://github.com/woodpecker-ci/woodpecker/blob/master/pipeline/backend/docker/docker.go#L39 hardcode the path.

[wxiaoguang]

I bypass this problem by setting:

    environment:
      - DOCKER_HOST=unix:///data-dind/dockerd/docker.sock
      - WOODPECKER_BACKEND=docker