docs: point out known issue with nginx/vboxfs

see https://github.com/docker/docker/issues/18666

Signed-off-by: Felix Geisendörfer <felix@debuggable.com>

docs/installation/mac.md

@@ -384,6 +384,10 @@ The next exercise demonstrates how to do this.
         $ docker stop mysite
         $ docker rm mysite
 
+> **Note**: There is a [known
+> issue](https://docs.docker.com/machine/drivers/virtualbox/#known-issues) that
+> may cause files shared with your nginx container to not update correctly as you
+> modify them on your host.
 
 ## Upgrade Docker Toolbox
 

docs/installation/windows.md

@@ -345,6 +345,11 @@ reported to you using:
 Typically, the IP is 192.168.59.103, but it could get changed by VirtualBox's
 DHCP implementation.
 
+> **Note**: There is a [known
+> issue](https://docs.docker.com/machine/drivers/virtualbox/#known-issues) that
+> may cause files shared with your nginx container to not update correctly as you
+> modify them on your host.
+
 ## Login with PUTTY instead of using the CMD
 
 Docker Machine generates and uses the public/private key pair in your