Docker fixes.

- Don't remove apk database, closes #1011.
- Install mosquitto_passwd, closes #1009.

docker/1.5/Dockerfile

@@ -51,13 +51,14 @@ RUN set -x && \
  mkdir -p /mosquitto/config /mosquitto/data /mosquitto/log && \
  install -d /usr/sbin/ && \
  install -s -m755 /build/src/mosquitto /usr/sbin/mosquitto && \
+ install -s -m755 /build/src/mosquitto_passwd /usr/bin/mosquitto_passwd && \
  install -m644 /build/mosquitto.conf /mosquitto/config/mosquitto.conf && \
  chown -R mosquitto:mosquitto /mosquitto && \
  apk --no-cache add \
  libuuid \
  libwebsockets && \
  apk del build-deps && \
- rm -rf /build /etc/apk /lib/apk
+ rm -rf /build
 
 VOLUME ["/mosquitto/data", "/mosquitto/log"]