Provide install instruction for mysql Docker container

[federico-razzoli]

After installing gcc, make and mysqlclient-dev, make still terminates with this error:

gcc -Wall -I/usr/include/mysql -I. -shared lib_mysqludf_sys.c -o /usr/lib/lib_mysqludf_sys.so
lib_mysqludf_sys.c:40:23: fatal error: my_global.h: No such file or directory

compilation terminated.
Makefile:4: recipe for target 'install' failed
make: *** [install] Error 1

Please provide isntructions.

[pavelsr]

Hey @federico-razzoli

You can check my Dockefile, I've made a pull requiest to add them into main repo