docs: Talk more about runtime properties, especially SELinux

Since SELinux is special let's elaborate on it more here.

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

Author: cgwalters

docs/src/building/bootc-runtime.md

@@ -61,3 +61,66 @@ Relevant links:
 - [CentOS Automotive SIG unattended updates](https://sigs.centos.org/automotive/building/unattended_updates/#watchdog-in-qemu)
   (note that as of right now, greenboot does not yet integrate with bootc)
 - <https://systemd.io/AUTOMATIC_BOOT_ASSESSMENT/>
+
+
+## Kernel
+
+When run as a container, the Linux kernel binary in
+`/usr/lib/modules/$kver/vmlinuz` is ignored.  It
+is only used when a bootc container is deployed
+to a physical or virtual machine.
+
+## Security properties
+
+When run as a container, the container runtime will by default apply
+various Linux kernel features such as namespacing to isolate
+the container processes from other system processes.
+
+None of these isolation properties apply when a bootc
+system is deployed.
+
+## SELinux
+
+Container runtimes such as `podman` and `docker` commonly
+apply a "coarse" SELinux policy to running containers.
+See [container-selinux](https://github.com/containers/container-selinux/blob/main/container_selinux.8).
+It is very important to understand that non-bootc base
+images do not (usually) have any embedded `security.selinux` metadata
+at all; all labels on the toplevel container image
+are *dynamically* generated per container invocation,
+and there are no individually distinct e.g. `etc_t` and
+`usr_t` types.
+
+In contrast, with the current OSTree backend for bootc,
+when the base image is built, label metadata is included
+in special metadata files in `/sysroot/ostree` that correspond
+to components of the base image.
+
+When a bootc container is deployed, the system
+will use these default SELinux labels.
+Further non-OSTree layers will be dynamically labeled
+using the base policy.
+
+Hence, at the current time it will *not* work to override
+the labels for files in derived layers by using e.g.
+
+```
+RUN semanage fcontext -a -t httpd_sys_content_t "/web(/.*)?"
+```
+
+(This command will write to `/etc/selinux/policy/$policy/`)
+
+It will *never* work to do e.g.:
+
+```
+RUN chcon -t foo_t /usr/bin/foo
+```
+
+Because the container runtime state will deny the attempt to
+"physically" set the `security.selinux` extended attribute.
+In contrast per above, future support for custom labeling
+will by default be done by customizing the policy file_contexts.
+
+References:
+
+- <https://github.com/ostreedev/ostree-rs-ext/issues/510>