docs: Add Docker Development info to CONTRIBUTING.md

CONTRIBUTING.md

@@ -18,3 +18,63 @@ that everyone should follow.
 [ideas]: https://github.com/recyclarr/recyclarr/discussions/categories/ideas
 [markdownlint]: https://github.com/DavidAnson/markdownlint
 [Code Cleanup]: https://www.jetbrains.com/help/rider/Code_Cleanup__Index.html
+
+## Docker Development
+
+The project's `Dockerfile` builds in two different mods: Development and production mode.
+
+### Production Build
+
+This is the default build type for the image. Given a specific version number, it will grab the
+appropriate binary from the corresponding Github Release and install that into the image.
+
+### Development Build
+
+This build allows you to make changes to Recyclarr and pull those into a local docker image build.
+This is especially useful if you want to test changes in Recyclarr before it is released, since the
+production mode of Recyclarr requires a Github release to pull from.
+
+To enable development builds, specify the build argument `BUILD_FROM_BRANCH`. The workflow I use
+goes something like this:
+
+1. Create a branch to work out of: `git checkout -b docker origin/master`.
+1. Make some C# code changes, commit, and **push to the remote repo**.
+1. Build the docker image locally:
+
+   ```sh
+   docker compose build --no-cache --progress plain --build-arg BUILD_FROM_BRANCH=docker
+   ```
+
+1. Execute it locally:
+
+   ```sh
+   docker compose run --rm recyclarr sonarr
+   ```
+
+### Build Arguments
+
+- `RELEASE_TAG` (Default: `latest`)<br>
+  The git tag (e.g. `v2.1.2`) that represents the Github Release in the upstream repository to grab
+  binaries from. May also use `latest` to represent the latest Github Release. Only used in
+  Production builds.
+
+- `TARGETPLATFORM` (Default: empty)<br>
+  Required. Specifies the runtime architecture of the image and is used to pull the correct prebuilt
+  binary from the specified Github Release. See the table in the Platform Support section for a list
+  of valid values.
+
+- `REPOSITORY` (Default: `recyclarr/recyclarr`)<br>
+  The Github repository name (either `user/repo` or `organization/repo` format) used to grab the
+  prebuilt release from (in Production builds) or to clone (in Development builds).
+
+- `BUILD_FROM_BRANCH` (Default: empty)<br>
+  If specified, Development build mode is enabled and the branch name specified here is used to
+  compile Recyclarr and use its final binary in the resulting docker image.
+
+### Platform Support
+
+| Docker Platform | Recyclarr Runtime  |
+| --------------- | ------------------ |
+| `linux/arm/v7`  | `linux-musl-arm`   |
+| `linux/arm64`   | `linux-musl-arm64` |
+| `linux/amd64`   | `linux-musl-x64`   |