Bump the base Docker image to alpine:3.8

Major change:
- PHP 7.2

Changelog:
- https://www.alpinelinux.org/posts/Alpine-3.8.0-released.html

Signed-off-by: VirtualTam <virtualtam@flibidi.net>

Dockerfile

@@ -25,7 +25,7 @@ RUN cd shaarli \
 
 # Stage 4:
 # - Shaarli image
-FROM alpine:3.7
+FROM alpine:3.8
 LABEL maintainer="Shaarli Community"
 
 RUN apk --update --no-cache add \