Upgrade to stretch, follow linter advices

Signed-off-by: Knut Ahlers <knut@ahlers.me>

Dockerfile

@@ -1,19 +1,23 @@
-FROM debian:jessie
+FROM debian:stretch
 
-ADD https://code.bitlbee.org/debian/release.key /tmp/bitlbee_release.key
-ADD https://jgeboski.github.io/obs.key /tmp/jgeboski.key
-ADD bitlbee.list /etc/apt/sources.list.d/bitlbee.list
+COPY bitlbee.list /etc/apt/sources.list.d/bitlbee.list
 
 RUN set -ex \
- && apt-key add /tmp/bitlbee_release.key \
- && apt-key add /tmp/jgeboski.key \
  && apt-get update \
- && apt-get install -y bitlbee bitlbee-steam \
- && apt-get -y autoremove
+ && apt-get install -y --no-install-recommends gnupg2 curl ca-certificates \
+ && curl -sSfL https://code.bitlbee.org/debian/release.key | apt-key add - \
+ && curl -sSfL https://jgeboski.github.io/obs.key | apt-key add - \
+ && apt-get update \
+ && apt-get install -y --no-install-recommends bitlbee bitlbee-steam \
+ && apt-get purge -y gnupg2 curl \
+ && apt-get -y autoremove \
+ && apt-get clean \
+ && rm -rf /var/lib/apt/lists/*
 
-ADD start.sh /usr/local/bin/start.sh
-ADD bitlbee.conf /etc/bitlbee/bitlbee.conf
-ADD motd.txt /etc/bitlbee/motd.txt
+
+COPY start.sh /usr/local/bin/start.sh
+COPY bitlbee.conf /etc/bitlbee/bitlbee.conf
+COPY motd.txt /etc/bitlbee/motd.txt
 
 USER bitlbee
 VOLUME ["/data", "/var/lib/bitlbee"]

bitlbee.list

@@ -1,2 +1,2 @@
-deb http://code.bitlbee.org/debian/master/jessie/amd64/ ./
+deb http://code.bitlbee.org/debian/master/stretch/amd64/ ./
 deb http://download.opensuse.org/repositories/home:/jgeboski/Debian_8.0 ./