docs: Add experimental section for bound images

Signed-off-by: Chris Kyrouac <[email protected]>

Author: ckyrouac

docs/src/SUMMARY.md

@@ -47,6 +47,7 @@
 # Experimental features
 
 - [bootc image](experimental-bootc-image.md)
+- [logically bound images](experimental-logically-bound-images.md)
 
 # More information
 

docs/src/experimental-logically-bound-images.md

@@ -0,0 +1,35 @@
+# Logically Bound Images
+
+Experimental features are subject to change or removal. Please
+do provide feedback on them.
+
+Tracking issue: <https://github.com/containers/bootc/issues/128>
+
+## About logically bound images
+
+This experimental feature enables an association of container "app" images to a bootc system image. A similar approach to this is [physically bound](https://github.com/containers/bootc/issues/644) images. There are some trade-offs between the two approaches. Some benefits of logically bound images are:
+
+- The bootc system image can be updated without re-downloading the app image bits.
+- The app images can be updated without modifying the bootc system image, this would be especially useful for development work
+
+## Using logically bound images
+
+Each image is defined in a [Podman Quadlet](https://docs.podman.io/en/latest/markdown/podman-systemd.unit.5.html) `.image` or `.container` file. An image is selected to be bound by creating a symlink in the `/usr/lib/bootc-experimental/bound-images.d` directory pointing to a `.image` or `.container` file. With these defined, during a `bootc upgrade` or `bootc switch` the bound images defined in the new bootc image will be automatically pulled via podman.
+
+An example Containerfile
+
+```Dockerfile
+FROM quay.io/myorg/myimage:latest
+
+COPY ./my-app.image /usr/share/containers/systemd/my-app.image
+COPY ./another-app.container /usr/share/containers/systemd/another-app.container
+
+RUN ln -s /usr/share/containers/systemd/my-app.image /usr/lib/bootc-experimental/bound-images.d/my-app.image && \
+    ln -s /usr/share/containers/systemd/my-app.image /usr/lib/bootc-experimental/bound-images.d/my-app.image
+```
+
+## Limitations
+
+- Currently, only the Image field of a `.image` or `.container` file is used to pull the image. Any other field is ignored.
+- There is no cleanup during rollback.
+- Images are subject to default garbage collection semantics; e.g. a background job pruning images without a running container may prune them. They can also be manually removed via e.g. podman rmi.