Update dev container (#6189)

Fixes https://github.com/hwchase17/langchain/issues/6172

As described in https://github.com/hwchase17/langchain/issues/6172, I'd
love to help update the dev container in this project.

**Summary of changes:**
- Dev container now builds (the current container in this repo won't
build for me)
- Dockerfile updates
- Update image to our [currently-maintained Python
image](https://github.com/devcontainers/images/tree/main/src/python/.devcontainer)
(`mcr.microsoft.com/devcontainers/python`) rather than the deprecated
image from vscode-dev-containers
- Move Dockerfile to root of repo - in order for `COPY` to work
properly, it needs the files (in this case, `pyproject.toml` and
`poetry.toml`) in the same directory
- devcontainer.json updates
- Removed `customizations` and `remoteUser` since they should be covered
by the updated image in the Dockerfile
     - Update comments
- Update docker-compose.yaml to properly point to updated Dockerfile
- Add a .gitattributes to avoid line ending conversions, which can
result in hundreds of pending changes
([info](https://code.visualstudio.com/docs/devcontainers/tips-and-tricks#_resolving-git-line-ending-issues-in-containers-resulting-in-many-modified-files))
- Add a README in the .devcontainer folder and info on the dev container
in the contributing.md

**Outstanding questions:**
- Is it expected for `poetry install` to take some time? It takes about
30 minutes for this dev container to finish building in a Codespace, but
a user should only have to experience this once. Through some online
investigation, this doesn't seem unusual
- Versions of poetry newer than 1.3.2 failed every time - based on some
of the guidance in contributing.md and other online resources, it seemed
changing poetry versions might be a good solution. 1.3.2 is from Jan
2023

---------

Co-authored-by: bamurtaugh <brmurtau@microsoft.com>
Co-authored-by: Samruddhi Khandale <samruddhikhandale@github.com>

.github/CONTRIBUTING.md

@@ -59,6 +59,8 @@ we do not want these to get in the way of getting good code into the codebase.
 
 ## 🚀 Quick Start
 
+> **Note:** You can run this repository locally (which is described below) or in a [development container](https://containers.dev/) (which is described in the [.devcontainer folder](https://github.com/hwchase17/langchain/tree/master/.devcontainer)).
+
 This project uses [Poetry](https://python-poetry.org/) as a dependency manager. Check out Poetry's [documentation on how to install it](https://python-poetry.org/docs/#installation) on your system before proceeding.
 
 ❗Note: If you use `Conda` or `Pyenv` as your environment / package manager, avoid dependency conflicts by doing the following first: