Resolve relative paths to absolute paths in command line arguments

The executor accepts a few arguments dockerfile, context, cache-dir and
digest-file that all represent paths. This commits allows those paths to
be relative to the working directory of the executor.

Fixes GoogleContainerTools#732 GoogleContainerTools#731 GoogleContainerTools#675

cmd/executor/cmd/root.go

@@ -81,6 +81,9 @@ var RootCmd = &cobra.Command{
 		if err := executor.CheckPushPermissions(opts); err != nil {
 			exit(errors.Wrap(err, "error checking push permissions -- make sure you entered the correct tag name, and that you are authenticated correctly, and try again"))
 		}
+		if err := resolveRelativePaths(); err != nil {
+			exit(errors.Wrap(err, "error resolving relative paths to absolute paths"))
+		}
 		if err := os.Chdir("/"); err != nil {
 			exit(errors.Wrap(err, "error changing to root dir"))
 		}
@@ -227,6 +230,37 @@ func resolveSourceContext() error {
 	return nil
 }
 
+func resolveRelativePaths() error {
+	optsPaths := []*string{
+		&opts.DockerfilePath,
+		&opts.SrcContext,
+		&opts.CacheDir,
+		&opts.TarPath,
+		&opts.DigestFile,
+	}
+
+	for _, p := range optsPaths {
+		// Skip empty path
+		if *p == "" {
+			continue
+		}
+		// Skip path that is already absolute
+		if filepath.IsAbs(*p) {
+			logrus.Debugf("Path %s is absolute, skipping", *p)
+			continue
+		}
+
+		// Resolve relative path to absolute path
+		var err error
+		relp := *p // save original relative path
+		if *p, err = filepath.Abs(*p); err != nil {
+			return errors.Wrapf(err, "Couldn't resolve relative path %s to an absolute path", *p)
+		}
+		logrus.Debugf("Resolved relative path %s to %s", relp, *p)
+	}
+	return nil
+}
+
 func exit(err error) {
 	fmt.Println(err)
 	os.Exit(1)