Bumps alpine from 3.13.3 to 3.13.4. Signed-off-by: dependabot[bot] <support@github.com>
@@ -1,4 +1,4 @@
-FROM alpine:3.13.3
+FROM alpine:3.13.4
ARG POSTFIX_PACKAGE_VERSION=3.5.9-r0
# http://www.postfix.org/postconf.5.html#tls_append_default_CA