Fix Jupyterlab docker build

[viniciusdc]

Fixes #1000

On December 16th code-server repo updated their install.sh file, and updated their download URLs, causing the postbuild phase for the Jupyterlab image to fail. This PR updates the download URL for their installer.

Also, on January 18th our docker build/push action got a new release including support for Handlebars, which conflicted with our main path for template images: ./qhub/templates/{{ cookiecutter.repo_directory }}, as they seem to have added some kind of parser for the handlers that broke our workflow.

Changes:

• Updates the snapshot URL for code-server bash installer
• Add symlink for ./qhub/templates/{{ cookiecutter.repo_directory }} --> ./qhub/templates/cookiecutter, and treeinspection over the symlink dir.

Types of changes

What types of changes does your code introduce?

Put an x in the boxes that apply

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds a feature)
• Breaking change (fix or feature that would cause existing features to not work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build related changes
• Other (please describe):

Testing

Requires testing

• Yes
• No

[iameskild]

Looks good to me! Thanks @viniciusdc :)