Add build instructions for the todored container image

todored/BUILD.md

@@ -21,3 +21,29 @@ $ cargo build --release
 ### Artifacts
 
 The built executable will be available in `target/release/todored`.
+
+## Building the container image
+
+### Prerequisites
+
+To build the `ghcr.io/steffo99/todocolors-red` container image, the following must be installed on your local machine:
+
+- [Docker Engine](https://docs.docker.com/engine/)
+
+Additionally, to build a multiplatform image, [buildx](https://docs.docker.com/engine/reference/commandline/buildx/) must be configured to use the `docker-container` driver, which can be used with:
+
+```console
+# docker buildx create --use
+```
+
+### Build and push
+
+To build the container image and push it to GitHub Containers, run:
+
+```console
+# docker buildx build . --tag ghcr.io/steffo99/todocolors-red --platform linux/amd64,linux/arm64/v8,linux/arm/v7 --push
+```
+
+### Artifacts
+
+The built image will be available at the `ghcr.io/steffo99/todocolors-red:latest` tag, both on the local machine and at the remote GitHub Containers repository.