quadlet: Add a network requirement on .image units

If a container unit starts on boot with a dependency on `default.target`
the image unit may start too soon, before network is ready. This cause
the unit to fail to pull the image.
- Add a dependency on `network-online.target` to make sure image pulls
don't fail.
See https://github.com/containers/podman/issues/21873

- Document the hardcoded dependency on `network-online.target` for images unit
and explain how it can be overriden if necessary.

- tests/e2e/quadlet: Add `assert-last-key-regex`

Required to test the `After=` override in [Unit] section
See https://github.com/containers/podman/pull/22057#issuecomment-2008959993

- quadlet/unitfile: add a prepenUnitLine method

Requirements on networks should be inserted at the top of the
section so the user can override them.

Signed-off-by: jbtrystram <jbtrystram@redhat.com>

docs/source/markdown/podman-systemd.unit.5.md

@@ -1333,6 +1333,11 @@ exists on the host, pulling it if needed.
 Using image units allows containers and volumes to depend on images being automatically pulled. This is
 particularly interesting when using special options to control image pulls.
 
+Note: The generated service have a dependency on `network-online.target` assuring the network is reachable if
+an image needs to be pulled.
+If the image service needs to run without available network (e.g. early in boot), the requirement can be
+overriden simply by adding an empty `After=` in the unit file. This will unset all previously set After's.
+
 Valid options for `[Image]` are listed below:
 
 | **[Image] options**                    | **podman image pull equivalent**                 |