Update README to account for new tar build output (bazelbuild#2123)

* Update README to account for new tar build output

The tar output location has been changed and is no longer stable. The old docs were not updated at the time of the change and the result is confusing build breakages. Until the original behavior is added back (not clear that it will happen) the README should document the new behavior.

See Issue bazelbuild#2014 for more info (this was closed as WAI), and [GoogleContainerTools PR#7251](GoogleContainerTools/skaffold#7251) for more context.

* Update README.md

add ".tar" to the query, it is needed!

Author: eap

README.md

@@ -245,16 +245,22 @@ performs the following steps:
 You can suppress this behavior by passing the single flag: `bazel run :foo -- --norun`
 
 Alternatively, you can build a `docker load` compatible bundle with:
-`bazel build my/image:helloworld.tar`.  This will produce the file:
-`bazel-bin/my/image/helloworld.tar`, which you can load into
-your local Docker client by running:
-`docker load -i bazel-bin/my/image/helloworld.tar`.  Building
-this target can be expensive for large images.
+`bazel build my/image:helloworld.tar`.  This will produce a tar file
+in your `bazel-out` directory that can be loaded into your local Docker
+client. Building this target can be expensive for large images. You will
+first need to query the ouput file location.
+
+```bash
+TARBALL_LOCATION=$(bazel cquery my/image:helloworld.tar \
+    --output starlark \
+    --starlark:expr="target.files.to_list()[0].path")
+docker load -i $TARBALL_LOCATION
+```
 
 These work with both `container_image`, `container_bundle`, and the
-`lang_image` rules.  For everything except
-`container_bundle`, the image name will be `bazel/my/image:helloworld`.
-For `container_bundle`, it will apply the tags you have specified.
+`lang_image` rules.  For everything except `container_bundle`, the image
+name will be `bazel/my/image:helloworld`. The `container_bundle` rule will
+apply the tags you have specified.
 
 ## Authentication