Set transport.Version via ldflags for release (#856)

For most things consuming this package, they will depend on an actual
released version of go-containerregistry. When we release
go-containerregistry itself, the binaries are built at the same version
as the library, which means that they don't depend on an actual released
version of go-containerregistry, so the version in debug info is just
"(devel)". For our own release, we will just set it to the tag.

.goreleaser.yml

@@ -16,6 +16,7 @@ builds:
   - -trimpath
   ldflags:
   - "-s -w -X github.com/google/go-containerregistry/cmd/crane/cmd.Version={{.Version}}"
+  - "-s -w -X github.com/google/go-containerregistry/pkg/v1/remote/transport.Version={{.Version}}"
   goarch:
     - amd64
     - arm
@@ -38,6 +39,7 @@ builds:
   - -trimpath
   ldflags:
   - "-s -w -X github.com/google/go-containerregistry/cmd/crane/cmd.Version={{.Version}}"
+  - "-s -w -X github.com/google/go-containerregistry/pkg/v1/remote/transport.Version={{.Version}}"
   goarch:
     - amd64
     - arm

pkg/v1/remote/transport/useragent.go

@@ -20,13 +20,19 @@ import (
 	"runtime/debug"
 )
 
+var (
+	// Version can be set via:
+	// -ldflags="-X 'github.com/google/go-containerregistry/pkg/v1/remote/transport.Version=$TAG'"
+	Version string
+
+	ggcrVersion = defaultUserAgent
+)
+
 const (
 	defaultUserAgent = "go-containerregistry"
 	moduleName       = "github.com/google/go-containerregistry"
 )
 
-var ggcrVersion = defaultUserAgent
-
 type userAgentTransport struct {
 	inner http.RoundTripper
 	ua    string
@@ -39,6 +45,11 @@ func init() {
 }
 
 func version() string {
+	if Version != "" {
+		// Version was set via ldflags, just return it.
+		return Version
+	}
+
 	info, ok := debug.ReadBuildInfo()
 	if !ok {
 		return ""