Update autoconf container instructions to use Docker registry

Recommend pulling the container from there instead of building it.

Also provide the (very slightly differing) instructions for using it
with podman.

build/tools/autoconf/README.md

@@ -8,12 +8,27 @@ generating it on another system often results in insignificant changes just due
 to a different version of autoconf being used. By using this container, we can
 use the same version everywhere.
 
-To use it, first build the container, then run it, see below.
+
+## Using Pre-Made Image
+
+To use this container, you can pull it from Docker registry and run it
+directly by executing the following command from the top-level wx directory:
+
+```shell
+$ podman run -v `pwd`:/wx --rm docker.io/vadz/autoconf-for-wx
+```
+
+or, if you prefer to use podman:
+
+```shell
+$ docker run -v `pwd`:/wx --rm vadz/autoconf-for-wx
+```
 
 
 ## Build
 
-Just run
+You can also build the container yourself easily using the Dockerfile in this
+directory, just run
 
 ```shell
 $ docker build -t autoconf-for-wx build/tools/autoconf
@@ -25,17 +40,6 @@ Successfully tagged autoconf-for-wx:latest
 ```
 
 
-## Run
-
-Use the following command to update `configure` from `configure.ac`:
-
-```shell
-$ docker run -v `pwd`:/wx --rm autoconf-for-wx
-```
-
-It should be run from the top-level wx directory.
-
-
 ## Updating autoconf version
 
 When we decide to switch to a newer autoconf version, we should update the