Pin single docker base image from commandline argument

Fetch the latest hash for a single docker base image supplied as
commandline argument.

Example:
```
$ dockpin docker-base ubuntu:16.04
ubuntu@sha256:0f71fa8d4d2d4292c3c617fda2b36f6dabe5c8b6e34c3dc5b0d17d4e704bd39c
```

docker.go

@@ -26,6 +26,15 @@ var (
 		RunE:         runDockerPin,
 	}
 
+	dockerResolveCmd = &cobra.Command{
+		Use:          "resolve [base-image]",
+		Example:      "resolve ubuntu:20.04",
+		Short:        "Prints the current digest of the given base image",
+		Args:         cobra.ExactArgs(1),
+		SilenceUsage: true,
+		RunE:         runDockerResolve,
+	}
+
 	dockerfile *string
 )
 
@@ -57,6 +66,21 @@ func runDockerPin(cmd *cobra.Command, args []string) error {
 	return ioutil.WriteFile(ifDash(*dockerfile, "/dev/stdout"), n, 0644)
 }
 
+func runDockerResolve(cmd *cobra.Command, args []string) error {
+	baseImageAndVersion := args[0]
+	dockerClient, err := docker.NewClientWithOpts(docker.FromEnv)
+	if err != nil {
+		return err
+	}
+	di, err := dockerClient.DistributionInspect(cmd.Context(), baseImageAndVersion, "")
+	if err != nil {
+		return err
+	}
+	hashedBase := baseImageAndVersion + "@" + string(di.Descriptor.Digest)
+	_, err = fmt.Println(hashedBase)
+	return err
+}
+
 func ifDash(fn string, repl string) string {
 	if fn == "-" {
 		return repl

main.go

@@ -19,6 +19,7 @@ func main() {
 	aptCmd.AddCommand(aptPinCmd)
 	aptCmd.AddCommand(aptInstallCmd)
 	dockerCmd.AddCommand(dockerPinCmd)
+	dockerCmd.AddCommand(dockerResolveCmd)
 	rootCmd.AddCommand(aptCmd)
 	rootCmd.AddCommand(dockerCmd)
 	if err := rootCmd.Execute(); err != nil {