add dockerfile, runs renew-dns.sh initially and foregrounds crond

Dockerfile

@@ -0,0 +1,9 @@
+FROM alpine
+
+ENV DOMAINS
+ENV TOKEN
+ENV VERBOSE=false
+
+COPY renew-dns.sh /etc/periodic/5min/renew-dns.sh
+
+CMD sh -c '/etc/periodic/5min/renew-dns.sh && crond -f -d 8'

renew-dns.sh

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+#!/bin/sh
+
+curl "https://www.duckdns.org/update?domains=$DOMAINS&token=$TOKEN&VERBOSE=$VERBOSE"