cli/connhelper: support overriding the docker binary over SSH

cli/connhelper/connhelper.go

@@ -5,13 +5,20 @@ import (
 	"context"
 	"net"
 	"net/url"
+	"os"
 	"strings"
 
 	"github.com/docker/cli/cli/connhelper/commandconn"
 	"github.com/docker/cli/cli/connhelper/ssh"
 	"github.com/pkg/errors"
 )
 
+const (
+	// DockerSSHRemoteBinaryEnv is the environment variable that can be used to
+	// override the default Docker binary called over SSH
+	DockerSSHRemoteBinaryEnv = "DOCKER_SSH_REMOTE_BINARY"
+)
+
 // ConnectionHelper allows to connect to a remote host with custom stream provider binary.
 type ConnectionHelper struct {
 	Dialer func(ctx context.Context, network, addr string) (net.Conn, error)
@@ -47,9 +54,10 @@ func getConnectionHelper(daemonURL string, sshFlags []string) (*ConnectionHelper
 		}
 		sshFlags = addSSHTimeout(sshFlags)
 		sshFlags = disablePseudoTerminalAllocation(sshFlags)
+		remoteDockerBinary := dockerSSHRemoteBinary()
 		return &ConnectionHelper{
 			Dialer: func(ctx context.Context, network, addr string) (net.Conn, error) {
-				args := []string{"docker"}
+				args := []string{remoteDockerBinary}
 				if sp.Path != "" {
 					args = append(args, "--host", "unix://"+sp.Path)
 				}
@@ -91,3 +99,15 @@ func disablePseudoTerminalAllocation(sshFlags []string) []string {
 	}
 	return append(sshFlags, "-T")
 }
+
+// dockerSSHRemoteBinary returns the binary to use when executing Docker
+// commands over SSH. It defaults to "docker" if the DOCKER_SSH_REMOTE_BINARY
+// environment variable is not set.
+func dockerSSHRemoteBinary() string {
+	value := os.Getenv(DockerSSHRemoteBinaryEnv)
+	if value == "" {
+		return "docker"
+	}
+
+	return value
+}

cli/connhelper/connhelper_test.go

@@ -63,3 +63,30 @@ func TestDisablePseudoTerminalAllocation(t *testing.T) {
 		})
 	}
 }
+
+func TestDockerSSHBinaryOverride(t *testing.T) {
+	testCases := []struct {
+		name     string
+		env      string
+		expected string
+	}{
+		{
+			name:     "Default",
+			env:      "",
+			expected: "docker",
+		},
+		{
+			name:     "Override",
+			env:      "other-binary",
+			expected: "other-binary",
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Setenv(DockerSSHRemoteBinaryEnv, tc.env)
+			result := dockerSSHRemoteBinary()
+			assert.Check(t, is.Equal(result, tc.expected))
+		})
+	}
+}