Fixes for ApacheDS 2.0.0.AM27 and Ubuntu:20.04

[dnrahamim]

This PR ensures that the Dockerfile can be built successfully. I made these changes because I was not able to build the base Image/Container upon cloning this repo.

Change Summary:

• Uses linux/amd64 emulation by default
• Uses ApacheDS version 2.0.0.AM27 (partly because version 2.0.0.AM26 couldn't be found at the previously hardcoded URL)
• Updates environment variables to use 2.0.0.AM28-SNAPSHOT (which is installed by 2.0.0.AM27)
• Fixes file paths broken by version upgrade
• Splits up a few build steps that have trouble completing on their own
• Installs python-ldap using pip (because apt-get wouldn't work). Installs necessary dependencies