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>

.dockerignore

@@ -0,0 +1,15 @@
+*.md
+*.xml
+*.yml
+*.txt
+*.html
+
+.gitignore
+.git/
+contrib/
+airsonic-main/
+!airsonic-main/target/airsonic.war
+subsonic-rest-api/
+airsonic-sonos-api/
+repo/
+target/