Bump alpine from 3.12.1 to 3.12.3

Bumps alpine from 3.12.1 to 3.12.3.

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

Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.12.1
+FROM alpine:3.12.3
 
 ARG POSTSRSD_PACKAGE_VERSION=1.6-r3
 RUN adduser -S postsrsd \