docker: allow successful compiling of nio4r gem dependency

- now seems to need linux-headers, see socketry/nio4r#225

docker/Dockerfile

@@ -6,7 +6,7 @@ ARG DYNDNSD_VERSION=3.3.0
 
 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 && \
+    apk --no-cache add --virtual .build-deps linux-headers ruby-dev build-base tzdata && \
     gem install --no-document dyndnsd -v ${DYNDNSD_VERSION} && \
     # set timezone to Berlin
     cp /usr/share/zoneinfo/Europe/Berlin /etc/localtime && \