fix: keep project build context as container path so local builder can stat it (#2314)

backend/internal/services/project_service.go

@@ -1759,14 +1759,6 @@ func resolveDockerfilePathInternal(svc composetypes.ServiceConfig) (string, erro
 	return dockerfilePath, nil
 }
 
-func translateBuildPathInternal(path string, pathMapper *projects.PathMapper) (string, error) {
-	if pathMapper == nil || strings.TrimSpace(path) == "" || !filepath.IsAbs(path) {
-		return path, nil
-	}
-
-	return pathMapper.ContainerToHost(path)
-}
-
 func buildArgsFromCompose(args map[string]*string) map[string]string {
 	buildArgs := map[string]string{}
 	for key, value := range args {
@@ -1895,14 +1887,18 @@ func (s *ProjectService) prepareServiceBuildRequest(
 		return imagetypes.BuildRequest{}, updatedSvc, updated, fmt.Errorf("service %s must define an image when push is enabled", serviceName)
 	}
 
+	// The build context (and any absolute Dockerfile path) is read locally by
+	// Arcane — both the docker provider (`archive.TarWithOptions`) and the
+	// buildkit provider (`SolveOpt.LocalDirs`) stream the directory contents
+	// to the daemon from the Arcane process's own filesystem. It must
+	// therefore stay as a container path; translating it to the host path
+	// (which is what bind mount sources need) makes `os.Stat` fail because
+	// the host path doesn't exist inside the Arcane container. See #2314.
+	// pathMapper is intentionally not consumed here for that reason.
 	contextDir, err := resolveBuildContextInternal(project.WorkingDir, updatedSvc, serviceName)
 	if err != nil {
 		return imagetypes.BuildRequest{}, updatedSvc, updated, err
 	}
-	contextDir, err = translateBuildPathInternal(contextDir, pathMapper)
-	if err != nil {
-		return imagetypes.BuildRequest{}, updatedSvc, updated, fmt.Errorf("translate build context for service %s: %w", serviceName, err)
-	}
 
 	dockerfileInline := updatedSvc.Build.DockerfileInline
 	if strings.TrimSpace(updatedSvc.Build.Dockerfile) != "" && strings.TrimSpace(dockerfileInline) != "" {
@@ -1915,10 +1911,6 @@ func (s *ProjectService) prepareServiceBuildRequest(
 		if err != nil {
 			return imagetypes.BuildRequest{}, updatedSvc, updated, err
 		}
-		dockerfilePath, err = translateBuildPathInternal(dockerfilePath, pathMapper)
-		if err != nil {
-			return imagetypes.BuildRequest{}, updatedSvc, updated, fmt.Errorf("translate Dockerfile path for service %s: %w", serviceName, err)
-		}
 	}
 
 	buildReq := imagetypes.BuildRequest{

backend/internal/services/project_service_test.go

@@ -1436,7 +1436,13 @@ func TestProjectService_PrepareServiceBuildRequest_MapsComposeFields(t *testing.
 	assert.Contains(t, req.ExtraHosts[0], "10.0.0.5")
 }
 
-func TestProjectService_PrepareServiceBuildRequest_UsesExecutorVisiblePaths(t *testing.T) {
+// TestProjectService_PrepareServiceBuildRequest_KeepsContainerPaths is a
+// regression test for #2314: Arcane's local build pipeline (the docker and
+// buildkit providers both read the build context via the Arcane process's own
+// filesystem) cannot use host paths, so prepareServiceBuildRequest must leave
+// the build context and any absolute Dockerfile path as container paths even
+// when the projects mount has a non-matching host prefix.
+func TestProjectService_PrepareServiceBuildRequest_KeepsContainerPaths(t *testing.T) {
 	svc := &ProjectService{}
 	proj := &composetypes.Project{WorkingDir: "/app/data/projects/demo", Name: "demo"}
 	pm := projects.NewPathMapper("/app/data/projects", "/docker-data/arcane/projects")
@@ -1461,8 +1467,42 @@ func TestProjectService_PrepareServiceBuildRequest_UsesExecutorVisiblePaths(t *t
 	)
 	require.NoError(t, err)
 
-	assert.Equal(t, "/docker-data/arcane/projects/demo", req.ContextDir)
-	assert.Equal(t, "/docker-data/arcane/projects/demo/Dockerfile.custom", req.Dockerfile)
+	assert.Equal(t, "/app/data/projects/demo", req.ContextDir)
+	assert.Equal(t, "/app/data/projects/demo/Dockerfile.custom", req.Dockerfile)
+}
+
+// TestProjectService_PrepareServiceBuildRequest_BuildDotKeepsContainerPath
+// reproduces the exact configuration from #2314: a compose file with
+// `build: .` next to its Dockerfile, on an installation where the projects
+// directory is bind-mounted from a different host path than the container
+// path. The resulting BuildRequest must point at the container path so the
+// local builder can stat / tar the directory.
+func TestProjectService_PrepareServiceBuildRequest_BuildDotKeepsContainerPath(t *testing.T) {
+	svc := &ProjectService{}
+	proj := &composetypes.Project{WorkingDir: "/app/data/projects/caddy", Name: "caddy"}
+	pm := projects.NewPathMapper("/app/data/projects", "/storage/volumes/arcane/projects")
+
+	serviceCfg := composetypes.ServiceConfig{
+		Name:  "caddy",
+		Image: "caddy",
+		Build: &composetypes.BuildConfig{
+			Context: ".",
+		},
+	}
+
+	req, _, _, err := svc.prepareServiceBuildRequest(
+		context.Background(),
+		"project-id",
+		proj,
+		"caddy",
+		serviceCfg,
+		ProjectBuildOptions{},
+		pm,
+	)
+	require.NoError(t, err)
+
+	assert.Equal(t, "/app/data/projects/caddy", req.ContextDir)
+	assert.Equal(t, "Dockerfile", req.Dockerfile)
 }
 
 func TestProjectService_PrepareServiceBuildRequest_UsesInlineDockerfile(t *testing.T) {