Edit devcontainer.json

When you start the dev container it's nice if you start in the /opt/epics directory and not in the /opt/epics/ioc directory. That way you can see the modules.yml file and other tools in the directory explorer.

Additionally, I added an env var which is useful when starting an IOC.

I ensure that git and cookie cutter are installed which are required to develop. It would be better if they were in the development image.

I also added the vs code epics extension