Dockerfile: fix java version mismatch by using maven image
Dockerfile: fix java version mismatch by using maven image

This should prevent java version mismatches error below and also reduces the
image size from 623MB to 440MB. It's still not around 300MB we had before
aaf6e20f9b02 (Dockerfile: use github, 2017-02-25), but reducing the size even
more can be done later. We are now using the `mvn` command directly so the
container does not need to install `make`.

Also added comments to the Dockerfile.  Hopefully this makes the file
more obvious for new docker users.

docker run --network="host" --add-host=hafza.local:127.0.0.1 nikita5/nikita-noark5-core
Exception in thread "main" java.lang.UnsupportedClassVersionError: no/arkivlab/hioa/nikita/webapp/run/N5CoreApp : Unsupported major.minor version 52.0
        at java.lang.ClassLoader.defineClass1(Native Method)
        at java.lang.ClassLoader.defineClass(ClassLoader.java:803)
        at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
        at java.net.URLClassLoader.defineClass(URLClassLoader.java:442)
        at java.net.URLClassLoader.access$100(URLClassLoader.java:64)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:354)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:348)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:347)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:425)
        at org.springframework.boot.loader.LaunchedURLClassLoader.loadClass(LaunchedURLClassLoader.java:89)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
        at org.springframework.boot.loader.MainMethodRunner.run(MainMethodRunner.java:46)
        at org.springframework.boot.loader.Launcher.launch(Launcher.java:87)
        at org.springframework.boot.loader.Launcher.launch(Launcher.java:50)
        at org.springframework.boot.loader.JarLauncher.main(JarLauncher.java:58)
Makefile:36: recipe for target 'docker_run' failed
make: *** [docker_run] Error 1

HiOA-ABI/nikita-noark5-core,HiOA-ABI/nikita-noark5-core,HiOA-ABI/nikita-noark5-core,HiOA-ABI/nikita-noark5-core