Align with knative/func#481

This PR aligns the docs to match updated ones on upstream - that rely
on systemd to manage secure UNIX socket.

Co-authored-by: Ashleigh Brennan <[email protected]>

Author: cardil

modules/serverless-functions-podman.adoc

@@ -7,21 +7,28 @@
 
 If you are using podman, you must run the following commands before getting started with {FunctionsProductName}:
 
-. Start the podman service that listens on port `1234`:
+. Start the podman service that serves the Docker API on a UNIX socket at `${XDG_RUNTIME_DIR}/podman/podman.sock`:
 +
 [source,terminal]
 ----
-$ podman system service --time=0 tcp:0.0.0.0:1234 & # let run in background or another terminal
+$ systemctl start --user podman.socket
 ----
++
+[NOTE]
+====
+On most systems, this socket is located at `/run/user/$(id -u)/podman/podman.sock`.
+====
 
 . Establish the environment variable that is used to build a function:
 +
 [source,terminal]
 ----
-$ export DOCKER_HOST=tcp://127.0.0.1:1234
+$ export DOCKER_HOST="unix://${XDG_RUNTIME_DIR}/podman/podman.sock"
 ----
+
+. Run the build command with `-v` to see verbose output. You should see a connection to your local UNIX socket:
 +
 [source,terminal]
 ----
-$ func build -v
+$ kn func build -v
 ----