Bump alpine from 3.12.3 to 3.13.0

Bumps alpine from 3.12.3 to 3.13.0.

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

Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.12.3
+FROM alpine:3.13.0
 
 ARG POSTSRSD_PACKAGE_VERSION=1.6-r3
 RUN adduser -S postsrsd \