Add Dockerfile

The Dockerfile is used to build a Docker image with all MaxScale
dependencied included. A short readme for using the image is attached.

docker/maxscale.cnf

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+# MaxScale documentation on GitHub:
+# https://github.com/mariadb-corporation/MaxScale/blob/develop/Documentation/Documentation-Contents.md
+
+# Complete list of configuration options:
+# https://github.com/mariadb-corporation/MaxScale/blob/develop/Documentation/Getting-Started/Configuration-Guide.md
+
+# Global parameters
+
+[maxscale]
+threads=2
+
+# This service enables the use of the MaxAdmin interface
+# MaxScale administration guide:
+# https://github.com/mariadb-corporation/MaxScale/blob/develop/Documentation/Reference/MaxAdmin.md
+
+[MaxAdmin-Service]
+type=service
+router=cli
+
+[MaxAdmin-Listener]
+type=listener
+service=MaxAdmin-Service
+protocol=maxscaled
+socket=default