Switch docker base image to alpine 3.9

alpine:3.4 only contains a docker image for arm64, so building for other
architectures doesn't work. The newest alpine relases, 3.9, is therefore
better.

cmd/loki/Dockerfile

-FROM       alpine:3.4
-RUN        apk add --update --no-cache ca-certificates && rm /var/cache/apk/*
+FROM       alpine:3.9
+RUN        apk add --update --no-cache ca-certificates
 COPY       loki /bin/loki
 COPY       loki-local-config.yaml /etc/loki/local-config.yaml
 EXPOSE     80

cmd/promtail/Dockerfile

-FROM       alpine:3.4
-RUN        apk add --update --no-cache ca-certificates && rm /var/cache/apk/*
+FROM       alpine:3.9
+RUN        apk add --update --no-cache ca-certificates
 ADD        promtail /usr/bin
 COPY       promtail-local-config.yaml /etc/promtail/local-config.yaml
 COPY       promtail-docker-config.yaml /etc/promtail/docker-config.yaml