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>

pkg/systemd/parser/unitfile.go

@@ -90,6 +90,11 @@ func (g *unitGroup) addLine(line *unitLine) {
 	g.lines = append(g.lines, line)
 }
 
+func (g *unitGroup) prependLine(line *unitLine) {
+	n := []*unitLine{line}
+	g.lines = append(n, g.lines...)
+}
+
 func (g *unitGroup) addComment(line *unitLine) {
 	g.comments = append(g.comments, line)
 }
@@ -923,6 +928,17 @@ func (f *UnitFile) PrependComment(groupName string, comments ...string) {
 	}
 }
 
+func (f *UnitFile) PrependUnitLine(groupName string, key string, value string) {
+	var group *unitGroup
+	if groupName == "" && len(f.groups) > 0 {
+		group = f.groups[0]
+	} else {
+		// Uses magic "" for first comment-only group if no other groups
+		group = f.ensureGroup(groupName)
+	}
+	group.prependLine(newUnitLine(key, value, false))
+}
+
 func (f *UnitFile) GetTemplateParts() (string, string) {
 	ext := filepath.Ext(f.Filename)
 	basename := strings.TrimSuffix(f.Filename, ext)