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/testdata/compose-full-spec.yaml

@@ -6,6 +6,7 @@ services:
       - ALL
     command: ["nginx", "updated", "command"]
     cpus: 0.5
+    pid: host
     deploy:
       update_config:
         order: stop-first