RegistryAuth: add method for building from auth token

src/main/java/com/spotify/docker/client/messages/RegistryAuth.java

@@ -144,15 +144,13 @@ public static RegistryAuth create(@JsonProperty("username") String username,
                                     @JsonProperty("identityToken") final String identityToken,
                                     @JsonProperty("auth") final String auth) {
 
+    final Builder builder;
     if (auth != null) {
-      final String[] authParams = Base64.decodeAsString(auth).split(":");
-
-      if (authParams.length == 2) {
-        username = authParams[0].trim();
-        password = authParams[1].trim();
-      }
+      builder = forAuthToken(auth);
+    } else {
+      builder = builder();
     }
-    return builder()
+    return builder
         .username(username)
         .password(password)
         .email(email)
@@ -161,6 +159,21 @@ public static RegistryAuth create(@JsonProperty("username") String username,
         .build();
   }
 
+  /** Construct a Builder based upon the "auth" field of the docker client config file. */
+  public static Builder forAuthToken(String auth) {
+    final String[] authParams = Base64.decodeAsString(auth).split(":");
+
+    if (authParams.length != 2) {
+      throw new IllegalArgumentException(
+          "auth token does not appear to be properly formatted after base64 decoding, "
+          + "did not find two values separated by a colon: "
+          + auth);
+    }
+    return builder()
+        .username(authParams[0].trim())
+        .password(authParams[1].trim());
+  }
+
   public static Builder builder() {
     return new AutoValue_RegistryAuth.Builder();
   }