examples: Fix docker-compose mount points

- Add /var/run/netns which is needed for named netns of
  cilium-health ep.
- Document netns related mounts.
- Remove unnecessary /run/docker/plugins mount from cilium-agent
  container, as cilium-agent does not need to access any plugin
  socket.

NOTE: we use `rshared` mount propagation for netns related mounts
to prevent from possibly leaking netns. See for more details:
moby/moby#32090.

Signed-off-by: Martynas Pumputis <m@lambda.lt>

examples/getting-started/docker-compose.yml

@@ -7,9 +7,11 @@ services:
     volumes:
       - /var/run/docker.sock:/var/run/docker.sock
       - /var/run/cilium:/var/run/cilium
-      - /run/docker/plugins:/run/docker/plugins
       - /sys/fs/bpf:/sys/fs/bpf
+      # To access Docker container netns:
       - /var/run/docker/netns:/var/run/docker/netns:rshared
+      # To create named netns for cilium-health endpoint:
+      - /var/run/netns:/var/run/netns:rshared
     network_mode: "host"
     cap_add:
       - "NET_ADMIN"