docs: add example Dockerfile

docs/Dockerfile

@@ -0,0 +1,15 @@
+FROM alpine:3.12
+
+EXPOSE 5353 8080
+
+ENV VERSION=3.0.0
+
+RUN apk --no-cache add openssl ca-certificates && \
+    apk --no-cache add ruby ruby-etc ruby-io-console ruby-json ruby-webrick && \
+    apk --no-cache add --virtual .build-deps ruby-dev build-base tzdata && \
+    gem install --no-document dyndnsd -v ${VERSION} && \
+    # set timezone to Berlin
+    cp /usr/share/zoneinfo/Europe/Berlin /etc/localtime && \
+    apk del .build-deps
+
+ENTRYPOINT ["dyndnsd", "/etc/dyndnsd/config.yml"]