quadlet: add HttpProxy option for Container sections

Add support for HttpProxy key in quadlet Container sections to control
proxy environment variable inheritance during image pulls and builds.

- HttpProxy=true enables proxy inheritance (default podman behavior)
- HttpProxy=false disables proxy inheritance
- When omitted, uses podman's default behavior

This addresses the need for declarative proxy configuration in IPv6-only
networks and other scenarios where proxy settings need to be controlled
at the container level without manual workarounds.

Fixes #26925

Signed-off-by: Ondřej Gajdušek <ogajduse@redhat.com>

pkg/systemd/quadlet/quadlet.go

@@ -109,6 +109,7 @@ const (
 	KeyHealthStartupTimeout  = "HealthStartupTimeout"
 	KeyHealthTimeout         = "HealthTimeout"
 	KeyHostName              = "HostName"
+	KeyHttpProxy             = "HttpProxy"
 	KeyImage                 = "Image"
 	KeyImageTag              = "ImageTag"
 	KeyInterfaceName         = "InterfaceName"
@@ -274,6 +275,7 @@ var (
 				KeyHealthStartupTimeout:  true,
 				KeyHealthTimeout:         true,
 				KeyHostName:              true,
+				KeyHttpProxy:             true,
 				KeyIP6:                   true,
 				KeyIP:                    true,
 				KeyImage:                 true,
@@ -676,6 +678,7 @@ func ConvertContainer(container *parser.UnitFile, isUser bool, unitsInfoMap map[
 	boolKeys := map[string]string{
 		KeyRunInit:         "--init",
 		KeyEnvironmentHost: "--env-host",
+		KeyHttpProxy:       "--http-proxy",
 		KeyReadOnlyTmpfs:   "--read-only-tmpfs",
 	}
 	lookupAndAddBoolean(container, ContainerGroup, boolKeys, podman)