Bump alpine from 3.13.3 to 3.13.4

Bumps alpine from 3.13.3 to 3.13.4.

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

Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.13.3
+FROM alpine:3.13.4
 
 ARG POSTSRSD_PACKAGE_VERSION=1.10-r0
 RUN adduser -S postsrsd \