Fix parsing panic if variables are not defined

lib/parser/dockerfile/base.go

@@ -70,6 +70,9 @@ func (d *baseDirective) replaceVars(vars map[string]string) error {
 	if err != nil {
 		return d.err(fmt.Errorf("Failed to replace variables in input: %s", err))
 	}
+	if len(replaced) == 0 {
+		return d.err(fmt.Errorf("Empty args after replacing variables: %s", err))
+	}
 	d.Args = replaced
 	return nil
 }

lib/parser/dockerfile/copy_test.go

@@ -34,6 +34,7 @@ func TestNewCopyDirective(t *testing.T) {
 		chown     string
 	}{
 		{"missing args", false, "copy ", nil, "", "", ""},
+		{"missing args after replace", false, "copy \\", nil, "", "", ""},
 		{"shell single source", true, `copy src dst`, []string{"src"}, "dst", "", ""},
 		{"shell multi source", true, `copy src1 src2 dst`, []string{"src1", "src2"}, "dst", "", ""},
 		{"shell substitution", true, `copy src1 ${prefix}src2 dst$suffix`, []string{"src1", "test_src2"}, "dst_test", "", ""},

lib/parser/dockerfile/from_test.go

@@ -43,6 +43,7 @@ func TestNewFromDirective(t *testing.T) {
 		{"bad 'as'", false, "from test_image:trusty sa test_alias", "", ""},
 		{"substitution", true, "from ${prefix}image:trusty as alias$suffix", "test_image:trusty", "alias_test"},
 		{"bad substitution", false, "from ${prefiximage:trusty as alias$suffix", "", ""},
+		{"empty substitution", false, "from ${0:+0}", "test_image", ""},
 	}
 
 	for _, test := range tests {