Support `copies` config entry for multi-stage builds

Summary:
Support a `copies` variant config entry that will result in a
multi-stage build, copying both shared library files and application
directory from a previously defined variant. This is essentially a
shorthand for two `artifacts` entries that are likely to be idiomatic to
multi-stage build/prod configurations.

Defined a new abstract `build.CopyFrom` instruction and corresponding
`docker.DockerCopyFrom` instruction and refactored the writing of these
Dockerfile lines to be accomplished using an `InstructionsForPhase`
method on `config.ArtifactsConfig`.

Implemented new support for `copies` configuration in
`config.VariantConfig` and an `InstructionsForPhase` method that returns
`build.CopyFrom` instructions for both the shared library and
application directories.

Fixes T174622
Depends on D759

Test Plan:
Run `go test ./...`. Run `blubber blubber.example.yaml production` and ensure
the right `COPY --from` lines are included for the final stage.

Reviewers: thcipriani, mobrovac, hashar, mmodell, #release-engineering-team

Reviewed By: thcipriani, #release-engineering-team

Tags: #release-engineering-team

Maniphest Tasks: T174622

Differential Revision: https://phabricator.wikimedia.org/D768

2 files changed, 15 insertions, 0 deletions

diff --git a/build/instructions.go b/build/instructions.go
index 5c12796..1b954a2 100644
--- a/build/instructions.go
+++ b/build/instructions.go
@@ -50,6 +50,15 @@ func (copy Copy) Compile() []string {
 	return append(quoteAll(copy.Sources), quote(copy.Destination))
 }
 
+type CopyFrom struct {
+	From string
+	Copy
+}
+
+func (cf CopyFrom) Compile() []string {
+	return append([]string{cf.From}, cf.Copy.Compile()...)
+}
+
 type Env struct {
 	Definitions map[string]string
 }
diff --git a/build/instructions_test.go b/build/instructions_test.go
index dbab4b5..8f9470c 100644
--- a/build/instructions_test.go
+++ b/build/instructions_test.go
@@ -36,6 +36,12 @@ func TestCopy(t *testing.T) {
 	assert.Equal(t, []string{`"source1"`, `"source2"`, `"dest"`}, i.Compile())
 }
 
+func TestCopyFrom(t *testing.T) {
+	i := build.CopyFrom{"foo", build.Copy{[]string{"source1", "source2"}, "dest"}}
+
+	assert.Equal(t, []string{"foo", `"source1"`, `"source2"`, `"dest"`}, i.Compile())
+}
+
 func TestEnv(t *testing.T) {
 	i := build.Env{map[string]string{
 		"fooname": "foovalue",