README: Add more information on base images

Came up in chat

Signed-off-by: Colin Walters <[email protected]>

Author: cgwalters

README.md

@@ -59,6 +59,10 @@ You can also build this project like any other Rust project, e.g. `cargo build -
 
 Many users will be more interested in base (container) images.
 
+To build base images "from scratch", see [docs/bootc-images.md].
+
+For pre-built base images:
+
 * [Fedora CoreOS](https://quay.io/repository/fedora/fedora-coreos) can be used as a base image; you will need to [enable bootc](https://github.com/coreos/rpm-ostree/blob/main/docs/bootc.md) there.
 * There is also an in-development [Project Sagano](https://gitlab.com/CentOS/cloud/sagano) for Fedora/CentOS.
 

docs/bootc-images.md

@@ -0,0 +1,58 @@
+# Understanding "bootc compatible" images
+
+At the current time, it does not work to just do:
+```
+FROM fedora
+RUN dnf -y install kernel
+```
+or
+```
+FROM debian
+RUN apt install kernel
+```
+
+And get an image compatible with bootc.  Supporting this
+is an eventual goal, however there are a few reasons why
+this doesn't yet work.  The biggest reason is SELinux
+labeling support; the underlying ostree stack currently
+handles this and requires that the "base image"
+have a pre-computed set of labels that can be used
+for any derived layers.
+
+# Building bootc compatible base images
+
+As a corollary to this, the build process
+for generating base images currently requires running
+through ostree tooling to generate an "ostree commit"
+which has some special formatting in the base image.
+
+However, the ostree usage is an implementation detail
+and the requirement on this will be lifted in the future.
+
+For example, the [rpm-ostree compose image](https://coreos.github.io/rpm-ostree/container/#creating-base-images)
+tooling currently streamlines this, operating just
+on a declarative input and writing to a registry.
+
+This is how the [Project Sagano](https://gitlab.com/CentOS/cloud/sagano)
+base images are built.
+
+# Deriving from existing base images
+
+However, it's important to emphasize that from one
+of these specially-formatted base images, every
+tool and technique for container building applies!
+In other words it will Just Work to do
+```
+FROM <bootc base image>
+RUN dnf -y install foo && dnf clean all 
+```
+
+## Using the `ostree container commit` command
+
+As an opt-in optimization today, you can also add `ostree container commit`
+as part of your `RUN` invocations.   This will perform early detection
+of some incompatibilities.
+
+However, its usage is not and will never be strictly required.
+
+