fix: default to unix scheme when DOCKER_HOST is a path

When DOCKER_HOST is set to a filesystem path without a scheme
(e.g. /var/run/docker.sock), parseDockerDaemonHost previously
defaulted to the "tcp" protocol, producing a confusing error:
"Cannot connect to the Docker daemon at tcp://localhost:2375/foo.sock"

Now, if the address starts with "/" or "./", the parser defaults
to "unix" instead of "tcp", which matches the user's intent and
produces a correct connection or a clear error.

Fixes #5846

Signed-off-by: Aaron Mark <64331623+amarkdotdev@users.noreply.github.com>

Author: amarkdotdev

opts/hosts.go

@@ -57,7 +57,14 @@ func parseDockerDaemonHost(addr string) (string, error) {
 	proto, host, hasProto := strings.Cut(addr, "://")
 	if !hasProto && proto != "" {
 		host = proto
-		proto = "tcp"
+		// If the address looks like a filesystem path, default to "unix"
+		// rather than "tcp" to avoid confusing errors when the user sets
+		// DOCKER_HOST=/path/to/sock without a unix:// prefix.
+		if strings.HasPrefix(host, "/") || strings.HasPrefix(host, "./") {
+			proto = "unix"
+		} else {
+			proto = "tcp"
+		}
 	}
 
 	switch proto {

opts/hosts_test.go

@@ -79,6 +79,8 @@ func TestParseDockerDaemonHost(t *testing.T) {
 		"unix://":                     "unix://" + defaultUnixSocket,
 		"fd://":                       "fd://",
 		"fd://something":              "fd://something",
+		"/var/run/docker.sock":        "unix:///var/run/docker.sock",
+		"./run/docker.sock":           "unix://run/docker.sock",
 		"localhost:":                  "tcp://localhost:2375",
 		"localhost:5555":              "tcp://localhost:5555",
 		"localhost:5555/path":         "tcp://localhost:5555/path",