Add support for Go build flags (#340)

There are use cases, where multiple Go build flags need to be set. However, the
environment variable to pass flags to Go build has some limits for `ldFlags`.

Add GoReleaser inspired configuration section to `.ko.yaml` to support setting
specific Go build and ldFlags to be used by the build. Like GoReleaser the
content of the configuration can use Go templates. Currently, only a section
for environment variables is included.

In order to reduce dependency overhead, only the respective config structs from
https://github.com/goreleaser/goreleaser/blob/master/pkg/config/config.go are
used internally to load from `.ko.yaml`.

pkg/commands/config_test.go

@@ -20,6 +20,7 @@ import (
 	"context"
 	"testing"
 
+	"github.com/google/ko/pkg/build"
 	"github.com/google/ko/pkg/commands/options"
 )
 
@@ -45,3 +46,34 @@ func TestOverrideDefaultBaseImageUsingBuildOption(t *testing.T) {
 		t.Errorf("got digest %s, wanted %s", gotDigest, wantDigest)
 	}
 }
+
+func TestCreateBuildConfigs(t *testing.T) {
+	compare := func(expected string, actual string) {
+		if expected != actual {
+			t.Errorf("test case failed: expected '%#v', but actual value is '%#v'", expected, actual)
+		}
+	}
+
+	buildConfigs := []build.Config{
+		{ID: "defaults"},
+		{ID: "OnlyMain", Main: "test"},
+		{ID: "OnlyMainWithFile", Main: "test/main.go"},
+		{ID: "OnlyDir", Dir: "test"},
+		{ID: "DirAndMain", Dir: "test", Main: "main.go"},
+	}
+
+	for _, b := range buildConfigs {
+		for importPath, buildCfg := range createBuildConfigs("../..", []build.Config{b}) {
+			switch buildCfg.ID {
+			case "defaults":
+				compare("github.com/google/ko", importPath)
+
+			case "OnlyMain", "OnlyMainWithFile", "OnlyDir", "DirAndMain":
+				compare("github.com/google/ko/test", importPath)
+
+			default:
+				t.Fatalf("unknown test case: %s", buildCfg.ID)
+			}
+		}
+	}
+}