mysql : add readme

pluie/alpine-mysql/README.md

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+# pluie/alpine-mysql
+
+This Image extend pluie/alpine with mysql (mariadb) 5.5.47
+
+
+## Docker image size
+
+- image ~ 160 MB
+
+## Docker image voumes
+
+__/var/lib/mysql__ : mysql database directory  
+__/dump__         : directory to store various mysql scripts  
+
+## Docker ENV variables
+
+__MYSQL_DATABASE__      : create specified database at startup  
+__MYSQL_USER__          : create specified user at startup (and grant all rights to __MYSQL_DATABASE__)  
+__MYSQL_PASSWORD__  
+__MYSQL_ROOT_PASSWORD__ : don't really need it. a random root password is generated if none  
+
+
+## Docker image usage
+
+```
+$ docker run --name mysql -p 3306 -v /home/docker/db/mysql:/var/lib/mysql -v /home/docker/db/dump:/dump -e MYSQL_DATABASE=mybase -e MYSQL_USER=dev -e MYSQL_PASSWORD=mysql -it pluie/alpine-mysqldocker
+```
+
+### root usage
+
+a root user is created at startup (if database directory is empty)  
+root login is permit only on localhost   
+to do things without files you can run :
+```
+$ docker exec -it mysql "mysql -uroot"
+```
+to do things with files you can log in to container :  
+```
+docker exec -it mysql bash
+```
+then  
+```
+$ mysql -uroot mybase < /dump/mydump.sql
+```
+etc.  
+
+
+### user usage
+
+locally (on host) you can run (with appropriate user : pass) :  
+```
+mysql -h ipmysqlcontainer -udev -pmysql
+```
+or if you want use mysql client of the container :
+```
+docker exec -it mysql "mysql -h ipmysqlcontainer -udev -pmysql"
+```
+
+## Link container
+
+to link this container to another, use the default docker behaviore :
+```
+docker run --name mycontainer --link mysql:db ...
+```