Jupyter Lab dockerfile ver 1.2

This release is just a tag for the Jupyter Lab Dockerfile. It does not contain any extra files or packages.

Since the 1.2 dockerfile, we support an installation script based on conda. This script only works in the following conditions:

1. The current python3 is managed by conda.
2. The current python3 is at least 3.6.
3. The dockerfile option JLAB_IMODE is configured as conda. This is the default configuration.

If either one of the above conditions is not satisfied, the installation would fall back to pip mode.