docs: update contribution instructions with instructions for Docker replacements (aws#34038)

### Reason for this change
aws-cdk supports Docker replacements such as Finch, but the CONTRIBUTING.md guidelines don't mention it.

### Description of changes
Add instructions for using Docker replacements.

### Describe any new or updated permissions being added
None

### Description of how you validated changes
Used the instructions myself.

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: crowecawcaw

CONTRIBUTING.md

@@ -222,8 +222,9 @@ The following tools need to be installed on your system prior to installing the
 - [Yarn >= 1.19.1, < 2](https://yarnpkg.com/lang/en/docs/install)
 - [.NET SDK >= 6.0.x](https://www.microsoft.com/net/download)
 - [Python >= 3.8.0, < 4.0](https://www.python.org/downloads/release/python-380/)
-- [Docker >= 19.03](https://docs.docker.com/get-docker/)
-  - the Docker daemon must also be running
+- Either [Docker >= 19.03](https://docs.docker.com/get-docker/), [Finch >= 0.3.0](https://runfinch.com/), or another Docker replacement
+  - If using a Docker replacement, the `CDK_DOCKER` environment variable must be set to the replacement command's name (e.g. `export CDK_DOCKER=finch`)
+  - The Docker or replacement daemon must be running
 - [git-lfs](https://docs.github.com/en/repositories/working-with-files/managing-large-files/installing-git-large-file-storage)
   - Without this, you'll get the message that the clone succeeded but the checkout failed when you initially clone the repo.