Add command hook.

Dockerfile

@@ -1,10 +1,9 @@
 FROM alpine:latest
 
-RUN apk update && apk upgrade \
-    && apk add \
-         curl \
-         certbot \
-         openssl
+RUN apk add \
+        curl \
+        certbot \
+        openssl
 
 RUN mkdir -p /etc/periodic/12h \
     && mkdir -p /etc/periodic/5min

certbot-duckdns

@@ -11,3 +11,7 @@ certbot certonly --manual --preferred-challenges dns-01 --keep \
     --agree-tos --no-eff-email --manual-public-ip-logging-ok \
     --manual-auth-hook=certbot-auth --manual-cleanup-hook=certbot-cleanup \
     "$@"
+
+if [ -n "$COMMAND" ]; then
+    /usr/bin/env sh -c "$COMMAND"
+fi