switch vlc to stretch-slim

Previously this Dockerfile was using buster, but would throw the error:
```
Could not find the Qt platform plugin "xcb" in ""
```

Switching to stretch solves the problem.

Similar to issue:
jessfraz#401

vlc/Dockerfile

@@ -9,7 +9,7 @@
 #	--name vlc \
 #	jess/vlc
 #
-FROM debian:buster-slim
+FROM debian:stretch-slim
 LABEL maintainer "Jessie Frazelle <jess@linux.com>"
 
 RUN apt-get update && apt-get install -y \