Update base image to Alpine 3.12.5

Alpine 3.12.4 uses a vulnerable version of libssl1.1
(CVE-2021-3449 and CVE-2021-3450), issues that are fixed in Alpine
3.12.5. This is not really a problem for Webdis since it doesn't use
SSL, but the vulnerability shows up on image scans and users who build
images with Webdis as the base image could be at risk if their own
changes depend on this library.

Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.12.4 AS stage
+FROM alpine:3.12.5 AS stage
 LABEL maintainer="Nicolas Favre-Felix <n.favrefelix@gmail.com>"
 
 RUN apk update && apk add wget make gcc libevent-dev msgpack-c-dev musl-dev bsd-compat-headers jq
@@ -9,7 +9,7 @@ RUN cd webdis-$(cat latest) && make && make install && cd ..
 RUN sed -i -e 's/"daemonize":.*true,/"daemonize": false,/g' /etc/webdis.prod.json
 
 # main image
-FROM alpine:3.12.4
+FROM alpine:3.12.5
 RUN apk update && apk add libevent msgpack-c redis       # Required dependencies
 RUN apk add libcrypto1.1                                 # Security updates
 COPY --from=stage /usr/local/bin/webdis /usr/local/bin/