build(deps): bump alpine from 3.11 to 3.12.1

Bumps alpine from 3.11 to 3.12.1.

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

Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.11
+FROM alpine:3.12.1
 
 ARG POSTFIX_PACKAGE_VERSION=3.4.9-r0
 # http://www.postfix.org/postconf.5.html#tls_append_default_CA