4 files changed, 30 insertions, 8 deletions
diff --git a/build/instructions.go b/build/instructions.go
index 295221e..0167a8a 100644
--- a/build/instructions.go
+++ b/build/instructions.go
@@ -81,17 +81,20 @@ func (copy Copy) Compile() []string {
 // CopyAs is a concrete build instruction for copying source
 // files/directories and setting their ownership to the given UID/GID.
 //
+// While it can technically wrap any build.Instruction, it is meant to be used
+// with build.Copy and build.CopyFrom to enforce file/directory ownership.
+//
 type CopyAs struct {
 	UID uint // owner UID
 	GID uint // owner GID
-	Copy
+	Instruction
 }
 
 // Compile returns the variant name unquoted and all quoted CopyAs instruction
 // fields.
 //
 func (ca CopyAs) Compile() []string {
-	return append([]string{fmt.Sprintf("%d:%d", ca.UID, ca.GID)}, ca.Copy.Compile()...)
+	return append([]string{fmt.Sprintf("%d:%d", ca.UID, ca.GID)}, ca.Instruction.Compile()...)
 }
 
 // CopyFrom is a concrete build instruction for copying source
diff --git a/build/instructions_test.go b/build/instructions_test.go
index 29508e2..77938dd 100644
--- a/build/instructions_test.go
+++ b/build/instructions_test.go
@@ -37,9 +37,25 @@ func TestCopy(t *testing.T) {
 }
 
 func TestCopyAs(t *testing.T) {
-	i := build.CopyAs{123, 124, build.Copy{[]string{"source1", "source2"}, "dest"}}
-
-	assert.Equal(t, []string{"123:124", `"source1"`, `"source2"`, `"dest"`}, i.Compile())
+	t.Run("wrapping Copy", func(t *testing.T) {
+		i := build.CopyAs{
+			123,
+			124,
+			build.Copy{[]string{"source1", "source2"}, "dest"},
+		}
+
+		assert.Equal(t, []string{"123:124", `"source1"`, `"source2"`, `"dest"`}, i.Compile())
+	})
+
+	t.Run("wrapping CopyFrom", func(t *testing.T) {
+		i := build.CopyAs{
+			123,
+			124,
+			build.CopyFrom{"foo", build.Copy{[]string{"source1", "source2"}, "dest"}},
+		}
+
+		assert.Equal(t, []string{"123:124", "foo", `"source1"`, `"source2"`, `"dest"`}, i.Compile())
+	})
 }
 
 func TestCopyFrom(t *testing.T) {
diff --git a/build/macros.go b/build/macros.go
index 5d3422e..08556d1 100644
--- a/build/macros.go
+++ b/build/macros.go
@@ -11,9 +11,10 @@ func ApplyUser(uid uint, gid uint, instructions []Instruction) []Instruction {
 	applied := make([]Instruction, len(instructions))
 
 	for i, instruction := range instructions {
-		if copy, iscopy := instruction.(Copy); iscopy {
-			applied[i] = CopyAs{uid, gid, copy}
-		} else {
+		switch instruction.(type) {
+		case Copy, CopyFrom:
+			applied[i] = CopyAs{uid, gid, instruction}
+		default:
 			applied[i] = instruction
 		}
 	}
diff --git a/build/macros_test.go b/build/macros_test.go
index e47cf8d..c5066a6 100644
--- a/build/macros_test.go
+++ b/build/macros_test.go
@@ -12,12 +12,14 @@ func TestApplyUser(t *testing.T) {
 	instructions := []build.Instruction{
 		build.Copy{[]string{"foo"}, "bar"},
 		build.Copy{[]string{"baz"}, "qux"},
+		build.CopyFrom{"foo", build.Copy{[]string{"a"}, "b"}},
 	}
 
 	assert.Equal(t,
 		[]build.Instruction{
 			build.CopyAs{123, 223, build.Copy{[]string{"foo"}, "bar"}},
 			build.CopyAs{123, 223, build.Copy{[]string{"baz"}, "qux"}},
+			build.CopyAs{123, 223, build.CopyFrom{"foo", build.Copy{[]string{"a"}, "b"}}},
 		},
 		build.ApplyUser(123, 223, instructions),
 	)