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

1 files changed, 8 insertions, 8 deletions

diff --git a/README.md b/README.md
index 38cdcae..b8018a7 100644
--- a/README.md
+++ b/README.md
@@ -14,8 +14,6 @@ running ad-hoc commands.
 base: debian:jessie
 apt:
   packages: [libjpeg, libyaml]
-node:
-  dependencies: true
 runs:
   in: /srv/service
   as: runuser
@@ -26,13 +24,18 @@ runs:
     BAR: baz
 
 variants:
-  development:
+  build:
     apt:
       packages: [libjpeg-dev, libyaml-dev]
+    node:
+      dependencies: true
+
+  development:
+    includes: [build]
     sharedvolume: true
 
   test:
-    includes: [development]
+    includes: [build]
     apt:
       packages: [chromium]
     entrypoint: [npm, test]
@@ -41,10 +44,7 @@ variants:
     base: debian:jessie-slim
     node:
       env: production
-    artifacts:
-      - from: test
-        source: /srv/service
-        destination: .
+    copies: test
     entrypoint: [node, server.js]
 ```