Faster docker builds with make -j

Build object files concurrently with `make -j`, applied to both build
steps. As a result the docker image builds almost twice as fast.

Dockerfile

@@ -5,7 +5,7 @@ RUN apk update && apk add wget make gcc libevent-dev msgpack-c-dev musl-dev open
 RUN wget -q https://api.github.com/repos/nicolasff/webdis/tags -O /dev/stdout | jq '.[] | .name' | head -1  | sed 's/"//g' > latest
 RUN wget https://github.com/nicolasff/webdis/archive/$(cat latest).tar.gz -O webdis-latest.tar.gz
 RUN tar -xvzf webdis-latest.tar.gz
-RUN cd webdis-$(cat latest) && make && make install && make clean && make SSL=1 && cp webdis /usr/local/bin/webdis-ssl && cd ..
+RUN cd webdis-$(cat latest) && make -j && make install && make clean && make -j SSL=1 && cp webdis /usr/local/bin/webdis-ssl && cd ..
 RUN sed -i -e 's/"daemonize":.*true,/"daemonize": false,/g' /etc/webdis.prod.json
 
 # main image