Include meta data as labels in Dockerfile output

Summary:
Certain meta data including the Blubber version and variant used at
invocation may be useful in downstream tracking.

Implemented abstract and Docker-specific instructions for labels and
modified the Docker compiler to include these instructions at the end of
the output for the final stage.

Depends on D816
Fixes T178022

Test Plan:
Run unit tests. Run `make && bin/blubber blubber.example.yaml production` and
verify that both `blubber.version` and `blubber.variant` labels are present
and accurate.

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

Reviewed By: thcipriani, #release-engineering-team

Tags: #release-engineering-team

Maniphest Tasks: T178022

Differential Revision: https://phabricator.wikimedia.org/D818

2 files changed, 41 insertions, 12 deletions

diff --git a/build/instructions.go b/build/instructions.go
index 38ea322..41c959d 100644
--- a/build/instructions.go
+++ b/build/instructions.go
@@ -64,20 +64,18 @@ type Env struct {
 }
 
 func (env Env) Compile() []string {
-	defs := make([]string, 0, len(env.Definitions))
-	names := make([]string, 0, len(env.Definitions))
-
-	for name := range env.Definitions {
-		names = append(names, name)
-	}
-
-	sort.Strings(names)
+	return compileSortedKeyValues(env.Definitions)
+}
 
-	for _, name := range names {
-		defs = append(defs, name+"="+quote(env.Definitions[name]))
-	}
+// Label represents a number of meta-data key/value pairs
+type Label struct {
+	Definitions map[string]string
+}
 
-	return defs
+// Compile returns the label key/value pairs as a number of `key="value"`
+// strings where the values are properly quoted
+func (label Label) Compile() []string {
+	return compileSortedKeyValues(label.Definitions)
 }
 
 type Volume struct {
@@ -88,6 +86,23 @@ func (vol Volume) Compile() []string {
 	return []string{quote(vol.Path)}
 }
 
+func compileSortedKeyValues(keyValues map[string]string) []string {
+	defs := make([]string, 0, len(keyValues))
+	names := make([]string, 0, len(keyValues))
+
+	for name := range keyValues {
+		names = append(names, name)
+	}
+
+	sort.Strings(names)
+
+	for _, name := range names {
+		defs = append(defs, name+"="+quote(keyValues[name]))
+	}
+
+	return defs
+}
+
 func quote(arg string) string {
 	return strconv.Quote(arg)
 }
diff --git a/build/instructions_test.go b/build/instructions_test.go
index acbdb7c..4098711 100644
--- a/build/instructions_test.go
+++ b/build/instructions_test.go
@@ -56,6 +56,20 @@ func TestEnv(t *testing.T) {
 	}, i.Compile())
 }
 
+func TestLabel(t *testing.T) {
+	i := build.Label{map[string]string{
+		"fooname": "foovalue",
+		"barname": "barvalue",
+		"quxname": "quxvalue",
+	}}
+
+	assert.Equal(t, []string{
+		`barname="barvalue"`,
+		`fooname="foovalue"`,
+		`quxname="quxvalue"`,
+	}, i.Compile())
+}
+
 func TestVolume(t *testing.T) {
 	i := build.Volume{"/foo/dir"}