Add project official docker image.

Tag for docker image is set from project version.
To build docker image for the project run:
- mvn package
- mvn -N dockerfile:build

Created docker image can be uploaded to the docker hub:
- mvn -N dockerfile:push

Signed-off-by: Yahor Berdnikau <egorr.berd@gmail.com>

Dockerfile

@@ -0,0 +1,27 @@
+FROM alpine:3.6
+
+LABEL description="Airsonic is a free, web-based media streamer, providing ubiquitious access to your music." \
+      url="https://github.com/airsonic/airsonic"
+
+ENV UID=1001 GID=1001 AIRSONIC_PORT=4040 AIRSONIC_DIR=/airsonic
+
+WORKDIR $AIRSONIC_DIR
+
+COPY dockerfiles/run.sh /usr/local/bin/run.sh
+COPY airsonic-main/target/airsonic.war airsonic.war
+
+RUN apk --no-cache add \
+    ffmpeg \
+    lame \
+    su-exec \
+    libressl \
+    ca-certificates \
+    tini \
+    openjdk8-jre \
+    && chmod +x /usr/local/bin/run.sh
+
+EXPOSE $AIRSONIC_PORT
+
+VOLUME $AIRSONIC_DIR/data $AIRSONIC_DIR/music $AIRSONIC_DIR/playlists $AIRSONIC_DIR/podcasts
+
+ENTRYPOINT ["run.sh"]