Recursive variant expansion

Summary:
Problem running blubber cfg.yml test with this config:

```
base: nodejs-slim
variants:
  build:
    base: nodejs-devel
  development:
    includes: [build]
    entrypoint: [npm, start]
  test:
    includes: [development]
    entrypoint: [npm, test]
```

yields a Dockerfile with `FROM nodejs-slim`; however, I expected that
the base would be `nodejs-devel` since `test` inherits from
`development` which inherits from `build`.

In order for this to work as expected we have to recursively expand
variants as in this patch.

Reviewers: dduvall, #release-engineering-team

Reviewed By: dduvall, #release-engineering-team

Tags: #release-engineering-team

Differential Revision: https://phabricator.wikimedia.org/D773

1 files changed, 15 insertions, 12 deletions

diff --git a/docker/compiler.go b/docker/compiler.go
index c9ca0fb..d7043a0 100644
--- a/docker/compiler.go
+++ b/docker/compiler.go
@@ -2,6 +2,7 @@ package docker
 
 import (
 	"bytes"
+	"log"
 	"strings"
 
 	"phabricator.wikimedia.org/source/blubber.git/build"
@@ -13,23 +14,25 @@ func Compile(cfg *config.Config, variant string) *bytes.Buffer {
 
 	vcfg, err := config.ExpandVariant(cfg, variant)
 
-	if err == nil {
-		// omit the main stage name unless multi-stage is required below
-		mainStage := ""
+	if err != nil {
+		log.Fatal(err)
+	}
 
-		// write multi-stage sections for each variant dependency
-		for _, stage := range vcfg.VariantDependencies() {
-			dependency, err := config.ExpandVariant(cfg, stage)
+	// omit the main stage name unless multi-stage is required below
+	mainStage := ""
 
-			if err == nil {
-				CompileStage(buffer, stage, dependency)
-				mainStage = variant
-			}
-		}
+	// write multi-stage sections for each variant dependency
+	for _, stage := range vcfg.VariantDependencies() {
+		dependency, err := config.ExpandVariant(cfg, stage)
 
-		CompileStage(buffer, mainStage, vcfg)
+		if err == nil {
+			CompileStage(buffer, stage, dependency)
+			mainStage = variant
+		}
 	}
 
+	CompileStage(buffer, mainStage, vcfg)
+
 	return buffer
 }