upgrade alpine base image v3.10->3.12.1 including postsrsd package upgrade v1.6-r1->1.6-r3 (changes below)

https://git.alpinelinux.org/aports/commit/community/postsrsd?id=c7a1f20490e2882233bfdfa682040d3cb0ffd967
https://git.alpinelinux.org/aports/commit/?id=26c15e1eb8f0128a46b5f302bdb90ff9d762618b

Dockerfile

@@ -1,7 +1,8 @@
-FROM alpine:3.10
+FROM alpine:3.12.1
 
+ARG POSTSRSD_PACKAGE_VERSION=1.6-r3
 RUN adduser -S postsrsd \
-    && apk add --no-cache postsrsd \
+    && apk add --no-cache postsrsd=$POSTSRSD_PACKAGE_VERSION \
     && mkdir -p /etc/postsrsd/secrets \
     && chown postsrsd /etc/postsrsd/secrets