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/README.md

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+# MariaDB MaxScale Docker image
+
+This Docker image runs MariaDB MaxScale. The MaxScale version used to build the
+image is an unreleased development build so this should not be used for a
+production system.
+
+## Usage
+
+MaxScale is a proxy so its configuration is dependent on the use case and there
+is no generally valid default. The configuration file inside the image (shown in
+[Default configuration](#default-configuration)) includes only the bare minimum
+to start MaxScale and has no useful services. The simplest way to add more to
+the configuration is to place another file (*my_config.cnf*) in a directory
+(*/my_dir/*) and then mount the directory to */etc/maxscale.cnf.d/* inside the
+container when starting it. MaxScale will add any files inside
+*/etc/maxscale.cnf.d/* to its configuration.
+
+```
+docker run --network host --rm -v /my_dir:/etc/maxscale.cnf.d/ maxscale
+```
+
+In the examples, the Docker network mode is set to *host* so that the container
+has full network access (`--network host`).
+
+To replace the configuration file completely, start MaxScale with `-f <path_to_config>`.
+Adding custom options removes all default options defined in the image. When
+adding new flags to MaxScale, one should add back `-l stdout` to print log to
+stdout.
+
+```
+docker run --network host --rm -v /my_dir:/container_dir maxscale -l stdout -f /container_dir/my_config.cnf
+```
+
+To save logs to */my_dir*, remove the `-l stdout` and set */container_dir* as
+log directory with the option `-L /container_dir`.
+
+```
+docker run --network host --rm -v /my_dir:/container_dir maxscale -L /container_dir -f /container_dir/my_config.cnf
+```
+
+## Default configuration
+
+```
+# 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
+```
+
+## Example configuration extension
+
+```
+# Server definitions
+# Set the address of the server to the network address of a MySQL server.
+
+[server1]
+type=server
+address=127.0.0.1
+port=3306
+protocol=MySQLBackend
+
+# Monitor for the servers
+# This will keep MaxScale aware of the state of the servers.
+# MySQL Monitor documentation:
+# https://github.com/mariadb-corporation/MaxScale/blob/develop/Documentation/Monitors/MySQL-Monitor.md
+
+[MySQL-Monitor]
+type=monitor
+module=mysqlmon
+servers=server1
+user=myuser
+passwd=mypwd
+monitor_interval=1000
+
+# Service definitions
+# Service Definition for a read-only service and a read/write splitting service.
+
+# ReadConnRoute documentation:
+# https://github.com/mariadb-corporation/MaxScale/blob/develop/Documentation/Routers/ReadConnRoute.md
+
+[Read-Only-Service]
+type=service
+router=readconnroute
+servers=server1
+user=myuser
+passwd=mypwd
+router_options=slave
+
+# ReadWriteSplit documentation:
+# https://github.com/mariadb-corporation/MaxScale/blob/develop/Documentation/Routers/ReadWriteSplit.md
+
+[Read-Write-Service]
+type=service
+router=readwritesplit
+servers=server1
+user=myuser
+passwd=mypwd
+max_slave_connections=100%
+
+# Listener definitions for the services
+# Listeners represent the ports the services will listen on.
+
+[Read-Only-Listener]
+type=listener
+service=Read-Only-Service
+protocol=MySQLClient
+port=4008
+
+[Read-Write-Listener]
+type=listener
+service=Read-Write-Service
+protocol=MySQLClient
+port=4006
+```