Update Dockerfile.stretch

* download packages.key from packages server

Dockerfile.stretch

@@ -8,7 +8,7 @@ RUN apt-get install -y --force-yes apt-transport-https curl gnupg1
 RUN echo "deb https://packages.spectrum.im/spectrum2/ stretch main" | tee -a /etc/apt/sources.list
 RUN echo "deb https://swift.im/packages/debian/stretch release main" | tee -a /etc/apt/sources.list
 RUN curl https://swift.im/keys/packages.key | apt-key add -
-RUN apt-key adv --keyserver hkp://pool.sks-keyservers.net --recv-keys 1AFDEA51
+RUN curl https://packages.spectrum.im/packages.key | apt-key add -
 RUN apt-get update -qq
 RUN apt-get install -y --force-yes build-essential git cmake prosody ngircd python-sleekxmpp libswiften-dev libprotobuf-dev protobuf-compiler libpurple-dev libglib2.0-dev libdbus-glib-1-dev liblog4cxx10-dev libpopt-dev libboost-dev libboost-signals-dev libboost-system-dev libboost-thread-dev libboost-locale-dev libboost-filesystem-dev libboost-program-options-dev libboost-regex-dev libboost-date-time-dev libcppunit-dev libcommuni-dev libminiupnpc-dev libnatpmp-dev libssl-dev libcurl4-openssl-dev libidn11-dev libxml2-dev libsqlite3-dev libqt4-dev libjsoncpp-dev
 RUN git clone git://github.com/SpectrumIM/spectrum2.git && cd spectrum2