Bump alpine from 3.10 to 3.12.1

Bumps alpine from 3.10 to 3.12.1.

Signed-off-by: dependabot[bot] <support@github.com>

Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.10
+FROM alpine:3.12.1
 
 RUN adduser -S postsrsd \
     && apk add --no-cache postsrsd \