feat: allow setting DOCKER_BIN path for docker execution

This change allows operators to specify a DOCKER_BIN environment variable. This will specify a binary to run when executing docker, which is useful in cases where the 'docker' command being run must be modified in a way that would otherwise be invasive to Dokku, but minimalistic if done within a wrapper.

plugins/network/network.go

@@ -132,7 +132,7 @@ func GetContainerPort(appName, processType string, isHerokuishContainer bool, co
 				break
 			}
 		}
-		cmd := sh.Command("docker", "port", containerID, port)
+		cmd := sh.Command(common.DockerBin(), "port", containerID, port)
 		cmd.Stderr = ioutil.Discard
 		b, err := cmd.Output()
 		if err == nil {