refactor: Modularize binding build functions

- Split the monolithic Build() function into focused helper functions.
- Add a TempFileManager for proper temporary file lifecycle management.

This refactoring is in preparation for implementing a local build API.

Signed-off-by: Jan Rodák <hony.com@seznam.cz>

internal/remote_build_helpers/utils.go

@@ -0,0 +1,82 @@
+package remote_build_helpers
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"os"
+
+	"github.com/sirupsen/logrus"
+)
+
+// TempFileManager manages temporary files created during image build.
+// It maintains a list of created temporary files and provides cleanup functionality
+// to ensure proper resource management.
+type TempFileManager struct {
+	files []string
+}
+
+func NewTempFileManager() *TempFileManager {
+	return &TempFileManager{}
+}
+
+func (t *TempFileManager) AddFile(filename string) {
+	t.files = append(t.files, filename)
+}
+
+func (t *TempFileManager) Cleanup() {
+	for _, file := range t.files {
+		if err := os.Remove(file); err != nil && !errors.Is(err, os.ErrNotExist) {
+			logrus.Errorf("Failed to remove temp file %s: %v", file, err)
+		}
+	}
+	t.files = t.files[:0] // Reset slice
+}
+
+// CreateTempFileFromReader creates a temporary file in the specified destination directory
+// with the given pattern, and copies content from the provided reader into the file.
+// The created temporary file is automatically added to the manager's cleanup list.
+//
+// Parameters:
+//   - dest: The directory where the temporary file should be created
+//   - pattern: The pattern for naming the temporary file
+//   - reader: The io.Reader from which to read content to write into the temporary file
+//
+// Returns:
+//   - string: The path to the created temporary file
+//   - error: Any error encountered during the operation
+func (t *TempFileManager) CreateTempFileFromReader(dest string, pattern string, reader io.Reader) (string, error) {
+	tmpFile, err := os.CreateTemp(dest, pattern)
+	if err != nil {
+		return "", fmt.Errorf("creating temp file: %w", err)
+	}
+	defer tmpFile.Close()
+
+	t.AddFile(tmpFile.Name())
+
+	if _, err := io.Copy(tmpFile, reader); err != nil {
+		return "", fmt.Errorf("copying stdin content: %w", err)
+	}
+	return tmpFile.Name(), nil
+}
+
+// CreateTempSecret creates a temporary copy of a secret file in the specified
+// context directory. The original secret file is copied to a new temporary file
+// which is automatically added to the manager's cleanup list.
+//
+// Parameters:
+//   - secretPath: The path to the source secret file to copy
+//   - contextDir: The directory where the temporary secret file should be created
+//
+// Returns:
+//   - string: The path to the created temporary secret file
+//   - error: Any error encountered during the operation
+func (t *TempFileManager) CreateTempSecret(secretPath, contextDir string) (string, error) {
+	secretFile, err := os.Open(secretPath)
+	if err != nil {
+		return "", fmt.Errorf("opening secret file %s: %w", secretPath, err)
+	}
+	defer secretFile.Close()
+
+	return t.CreateTempFileFromReader(contextDir, "podman-build-secret-*", secretFile)
+}