trim quotes from copy from/to paths (#283)

lib/builder/step/add_copy_step.go

@@ -64,6 +64,11 @@ func newAddCopyStep(
 	directive Directive, args, chown, fromStage string,
 	fromPaths []string, toPath string, commit bool) (*addCopyStep, error) {
 
+	toPath = strings.Trim(toPath, "\"'")
+	for i := range fromPaths {
+		fromPaths[i] = strings.Trim(fromPaths[i], "\"'")
+	}
+
 	if len(fromPaths) > 1 && !(strings.HasSuffix(toPath, "/") || toPath == "." || toPath == "..") {
 		return nil, fmt.Errorf("copying multiple source files, target must be a directory ending in \"/\"")
 	}

lib/builder/step/add_copy_step_test.go

@@ -44,3 +44,13 @@ func TestContextDirs(t *testing.T) {
 	require.Equal("stage", stage)
 	require.Len(paths, 1)
 }
+
+func TestTrimmingPaths(t *testing.T) {
+	require := require.New(t)
+
+	ac, err := newAddCopyStep(Copy, "", "", "", []string{"\"/from/path\""}, "\"/to/path\"", false)
+	require.NoError(err)
+
+	require.Equal("/from/path", ac.fromPaths[0])
+	require.Equal("/to/path", ac.toPath)
+}