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>

.devcontainer/devcontainer.json

@@ -1,24 +1,26 @@
 // For format details, see https://aka.ms/devcontainer.json. For config options, see the
-// README at: https://github.com/devcontainers/templates/tree/main/src/docker-existing-dockerfile
+// README at: https://github.com/devcontainers/templates/tree/main/src/docker-existing-docker-compose
 {
-	"dockerComposeFile": "./docker-compose.yaml",
-	"service": "langchain",
-	"workspaceFolder": "/workspaces/langchain",
+	// Name for the dev container
 	"name": "langchain",
-	"customizations": {
-		"vscode": {
-			"extensions": [   
-				"ms-python.python"
-			],
-			"settings": {
-				"python.defaultInterpreterPath": "/home/vscode/langchain-py-env/bin/python3.11"
-			}
-		}
-		
-	},
 
-	// Features to add to the dev container. More info: https://containers.dev/features.
-	"features": {},
+	// Point to a Docker Compose file
+	"dockerComposeFile": "./docker-compose.yaml",
+
+	// Required when using Docker Compose. The name of the service to connect to once running
+	"service": "langchain",
+
+	// The optional 'workspaceFolder' property is the path VS Code should open by default when
+	// connected. This is typically a file mount in .devcontainer/docker-compose.yml
+	"workspaceFolder": "/workspaces/${localWorkspaceFolderBasename}",
+
+	// Prevent the container from shutting down
+	"overrideCommand": true
+
+	// Features to add to the dev container. More info: https://containers.dev/features
+	// "features": {
+	// 	"ghcr.io/devcontainers-contrib/features/poetry:2": {}
+	// }
 
 	// Use 'forwardPorts' to make a list of ports inside the container available locally.
 	// "forwardPorts": [],
@@ -26,8 +28,9 @@
 	// Uncomment the next line to run commands after the container is created.
 	// "postCreateCommand": "cat /etc/os-release",
 
-	// Uncomment to connect as an existing user other than the container default. More info: https://aka.ms/dev-containers-non-root.
-	// "remoteUser": "devcontainer"
-	"remoteUser": "vscode",
-	"overrideCommand": true
+	// Configure tool-specific properties.
+	// "customizations": {},
+
+	// Uncomment to connect as root instead. More info: https://aka.ms/dev-containers-non-root.
+	// "remoteUser": "root"
 }