feat: support PID namespace mode in Compose (#276)

This adds `pid: host` compose key support.

Modelled after #238.

See: #237

Signed-off-by: Miek Gieben <miek@miek.nl>
Co-authored-by: Pasha Sviderski <me@psviderski.name>

pkg/api/service_test.go

@@ -206,6 +206,7 @@ func TestContainerSpec_Clone(t *testing.T) {
 	original := ContainerSpec{
 		CapAdd:     []string{"NET_ADMIN"},
 		CapDrop:    []string{"ALL"},
+		Pid:        container.PidMode("host"),
 		Command:    []string{"sh", "-c", "echo hello"},
 		Entrypoint: []string{"/bin/bash"},
 		Env: EnvVars{