Bump alpine to 3.11

There's a new release which appears to have no apparent down-
sides or breakages.

https://www.alpinelinux.org/posts/Alpine-3.11.2-released.html

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

sample-functions/gif-maker/Dockerfile

@@ -1,7 +1,12 @@
 FROM functions/alpine:latest
 
+USER root
+
 RUN apk add ffmpeg gifsicle
 WORKDIR /root/
 COPY entry.sh   .
 ENV fprocess="./entry.sh"
+
+USER 1000
+
 CMD ["fwatchdog"]