Added Dockerfile, test script and README

Dockerfile

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+FROM mariadb
+
+RUN apt-get update && \
+	apt-get install -y \
+        gcc \
+        libmysqlclient-dev \
+        nano \
+        && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /root/lib_mysqludf_sys
+
+COPY lib_mysqludf_sys.c ./
+COPY lib_mysqludf_sys.sql /docker-entrypoint-initdb.d/.
+COPY hello_world.sh /home/mysql/hello_world.sh
+
+RUN gcc -Wall -I/usr/include/mysql -I. -shared lib_mysqludf_sys.c -o $(mysql_config --plugindir)/lib_mysqludf_sys.so

README.md

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+![docker-cloud-automated](https://img.shields.io/docker/cloud/automated/pavelsr/mysqludf_sys)
+![docker-cloud-build](https://img.shields.io/docker/cloud/build/pavelsr/mysqludf_sys)
+![docker-stars](https://img.shields.io/docker/stars/pavelsr/mysqludf_sys)
+![docker-pulls](https://img.shields.io/docker/pulls/pavelsr/mysqludf_sys)
+
+<!-- TOC depthFrom:1 depthTo:6 withLinks:1 updateOnSave:1 orderedList:0 -->
+
+- [DESCRIPTION](#description)
+- [FUNCTIONS](#functions)
+	- [sys_eval](#syseval)
+	- [sys_exec](#sysexec)
+	- [sys_get](#sysget)
+	- [sys_set](#sysset)
+- [INSTALLATION](#installation)
+- [DOCKER IMAGE](#docker-image)
+	- [Build](#build)
+	- [Run](#run)
+- [EXAMPLES](#examples)
+- [SECUIRITY PRECAUTIONS](#secuirity-precautions)
+
+<!-- /TOC -->
+
+# DESCRIPTION
+
+Functions that allow to interact with the operating system
+
+# FUNCTIONS
+
+## sys_eval
+
+executes an arbitrary command, and returns it's output
+
+syntax: `sys_eval(arg1)`
+
+## sys_exec
+
+executes an arbitrary command, and returns it's exit code
+
+syntax: `sys_exec(arg1)`
+
+## sys_get
+
+gets the value of an environment variable
+
+syntax: `sys_get(arg1)`
+
+## sys_set
+
+create an environment variable, or update the value of an existing environment variable
+
+syntax: `sys_set(arg1, arg2)`
+
+arg1 - name of an environment value
+
+arg2 - expression that contains the value that is to be assigned to the environment variabl
+
+# INSTALLATION
+
+On runnning MySQL server just run `install.sh` script (don't forget to change here mysql root login and password)
+
+# DOCKER IMAGE
+
+## Build
+
+```
+docker build -t mysqludf/sys .
+```
+
+## Run
+
+```
+docker run -p 3306:3306 -e MYSQL_ROOT_PASSWORD=my-secret-pw -d mysqludf/sys
+```
+
+# EXAMPLES
+
+```
+SELECT sys_eval('id')
+SELECT sys_eval('echo $HOME')
+SELECT sys_eval('~/hello_world.sh')
+SELECT sys_eval('bash /root/lib_mysqludf_sys/hello_world.sh')
+```
+
+# SECUIRITY PRECAUTIONS
+
+UDFs are available to all database users - you cannot grant EXECUTE privileges for them.
+As the commandstring passed to `sys_exec` or `sys_eval` can do pretty much everything, 
+exposing the function poses a very real security hazard.
+
+Even for a benign user, it is possible to accidentally do a lot of damage with it.
+The call will be executed with the privileges of the os user that runs MySQL, 
+so it is entirely feasible to delete MySQL's data directory, or worse.

hello_world.sh

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+#!/usr/bin/env sh
+whoami
+echo "Hello world"