Update alpine base image in Dockerfile to 3.11

When building the extended version of Hugo using the Dockerfile and
`--build-arg HUGO_BUILD_TAGS=extended`, the obtained Docker container is
broken, because the source is build under alpine 3.11 and the compiled
binary is copied to an image based on alpine 3.10. This problem was most
likely introduced due to an update of the golang base image.

This commit changes the base image from alpine:3.10 to alpine:3.11,
fixing extended version builds.

Dockerfile

@@ -26,7 +26,7 @@ RUN mage hugo && mage install
 
 # ---
 
-FROM alpine:3.10
+FROM alpine:3.11
 
 COPY --from=build /go/bin/hugo /usr/bin/hugo