Deprecate Dev Dockerfile, Update README, Relock Pipfile, Add psql to Dev Container

[shiva-menta]

Various environment changes to reflect the current state of PCX backend:

• Deprecating Dev Dockerfile – Abandoning this in favor of developing in Dev Containers. Old Dockerfile.dev also was pinned to an old version of Python.
• Updating README – Removing instructions related to Dockerfile.dev, adding Dev Container-specific instructions, and cleaning up the content a bit.
• Relocking Pipfile – Resolving lingering versioning issues when running the backend for the first time.
• Add psql to Dev Container – We previously didn't install postgresql-client within the Dev Container, but this is needed for loading PCX data into database. Based on Dev Container ubuntu version, psql is outdated at version 14 compared to new pinned version of postgres at version 15.8 (closest version to our current prod database), but this should be okay.

[el-agua]

lgtm