Be more explicit about language versions

[skylarmb]

• Adds more explicit docker base image versions to avoid breaking changes
• Adds language version files like .python-version, etc for use with nvm, rbenv, pyenv etc.
• Adds the engines sections to the node backend and frontend to throw an error if people try to use npm version 7 or an unsupported version of node.
• Updates readme with specific language versions

[lindawoo-plaid]

LGTM (I'm unfamiliar with the "engines" and what it is used for, but I will investigate...)