Feature/docker autofetch img

[AbdullahGheith]

Updated the dockerfile and added a custom entrypoint script.

The purpose is:

• Make the installation easier. Theres no need for the host to have qemu utils installed because the script can download the latest iotgoat image directly from github using the latest url construction. It will then convert it to qcow2 and use that.
• If the user is still interested in using a specific version, they can still do that by placing the qcow2 file in the docker folder.
• Its also possible to simply place the img.gz file in the same directory, and the script will convert it to qcow2.

[AbdullahGheith]

@scriptingxss can we get this merged