fix(docker): stop panicking when the docker context can't be read

FindDockerDriverBuilder read the stdout of `docker context inspect`
without checking its error, and Run returns a nil result when the
command does not start or the context is cancelled, so a Feature build
on a docker-container builder could end in a nil pointer panic. A
context that can't be read now counts as no current context, and the
first docker-driver builder is still chosen, as when `buildx ls` names
no builder for the current context.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: aledbf

internal/docker/argbuilders_test.go

@@ -23,6 +23,7 @@ type cannedResponse struct {
 	stdout []byte
 	stderr []byte
 	code   int
+	err    error
 }
 
 func (r *recordingRunner) Run(_ context.Context, stream io.Writer, name string, args ...string) ([]byte, []byte, int, error) {
@@ -34,7 +35,7 @@ func (r *recordingRunner) Run(_ context.Context, stream io.Writer, name string,
 			_, _ = stream.Write(resp.stdout)
 			_, _ = stream.Write(resp.stderr)
 		}
-		return resp.stdout, resp.stderr, resp.code, nil
+		return resp.stdout, resp.stderr, resp.code, resp.err
 	}
 	return nil, nil, 0, nil
 }

internal/docker/builder_detect_held_test.go

@@ -1,6 +1,7 @@
 package docker
 
 import (
+	"errors"
 	"testing"
 
 	"github.com/devcontainers/cli/internal/log"
@@ -22,3 +23,14 @@ func TestBuilderDetectionIgnoresAFailedBuildx(t *testing.T) {
 		t.Errorf("FindDockerDriverBuilder after a failed ls = %q, want empty", got)
 	}
 }
+
+func TestFindDockerDriverBuilderWhenTheContextCannotBeRead(t *testing.T) {
+	rr := &recordingRunner{responses: []cannedResponse{
+		{err: errors.New("exec docker: no such file")},
+		{stdout: []byte("NAME DRIVER\nremote* docker-container running\ndefault docker running\n")},
+	}}
+	c := &Client{DockerPath: "docker", Log: log.Null, Runner: rr}
+	if got := c.FindDockerDriverBuilder(t.Context()); got != "default" {
+		t.Errorf("FindDockerDriverBuilder with no current context = %q, want the first docker-driver builder", got)
+	}
+}

internal/docker/client.go

@@ -218,8 +218,10 @@ func (c *Client) DetectActiveBuilder(ctx context.Context) *BuilderInfo {
 }
 
 func (c *Client) FindDockerDriverBuilder(ctx context.Context) string {
-	ctxRes, _ := c.Run(ctx, "context", "inspect", "--format", "{{.Name}}")
-	currentContext := strings.TrimSpace(string(ctxRes.Stdout))
+	currentContext := ""
+	if ctxRes, err := c.Run(ctx, "context", "inspect", "--format", "{{.Name}}"); err == nil && ctxRes.ExitCode == 0 {
+		currentContext = strings.TrimSpace(string(ctxRes.Stdout))
+	}
 
 	res, err := c.Run(ctx, "buildx", "ls")
 	if err != nil || res.ExitCode != 0 {