Connecting docker spring image to mysql

So I am very new to kotlin. I am learning spring boot dockerizing.
I have existing spring boot image. I want to connect it with mysql.
So here is my github:
What goes wrong mates :(. I tried to connect with docker-compose. I also tried to enter:

docker run -p 8080:8080 --name dockerdb --link mysqldemo:mysql -d docker

But there is always the same problem:

Caused by: com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure

What goes wrong?

Source: StackOverflow