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/client/compose/service_test.go

@@ -127,6 +127,7 @@ func TestServiceSpecFromCompose(t *testing.T) {
 								"max-file": "3",
 							},
 						},
+						Pid:        "host",
 						Privileged: true,
 						PullPolicy: api.PullPolicyAlways,
 						Resources: api.ContainerResources{