update: add dockerfile for OSP 0.7

New Dokerfile and entrypoint.sh for OSP 0.7 with ejabberd.

Notes about changes:

• I have created a new v0.7 directory at the same level as Beta6. I don't know if your intention is to keep several directories with the successive versions or only one version. In the second case, I think the correct thing to do would be to rename the Beta6 directory in a more generic way.
• The environment variable OSP_SERVER_ADDRESS should be mandatory because ejabberd must be configured with this value, before any interaction with the user.
• The ejabberd is installed in the /usr/local/ejabberd directory, similar to the nginx that is installed in /usr/local/nginx.
• I did a refactoring of the Dockerfile to adapt it to a multistage build. This considerably reduces the size of the image (Beta6 -> 788MB, v0.7 -> 386MB) and the amount of layers in the final image.
• I set the alpine version to 3.12 instead of using latest. I think is better to set the version to have better control of the built image.

Let me know about any modifications you see necessary in this implementation to adapt it.