docs: Add a new bootc-runtime.md

Let's clarify this more; it's touched on in the main docs
but worth elaborating on.

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

Author: cgwalters

docs/src/SUMMARY.md

@@ -9,6 +9,7 @@
 # Building images
 
 - [Building images](building/guidance.md)
+- [Container runtime vs bootc runtime](building/bootc-runtime.md)
 - [Users, groups, SSH keys](building/users-and-groups.md)
 - [Secrets](building/secrets.md)
 - [Management Services](building/management-services.md)

docs/src/building/bootc-runtime.md

@@ -0,0 +1,47 @@
+
+# Container runtime vs "bootc runtime"
+
+Fundamentally, `bootc` reuses the [OCI image format](https://github.com/opencontainers/image-spec)
+as a way to transport serialized filesystem trees with included metadata such as a `version`
+label, etc.
+
+However, `bootc` generally ignores the [Container configuration](https://github.com/opencontainers/image-spec/blob/main/config.md)
+section at runtime today.
+
+Container runtimes like `podman` and `docker` of course *will* interpret this metadata
+when running a bootc container image as a container.
+
+## Labels
+
+A key aspect of OCI is the ability to use standardized (or semi-standardized)
+labels.  The are stored and rendered by `bootc`; especially the
+`org.opencontainers.image.version` label.
+
+## Example ignored runtime metadata, and recommendations
+
+### `ENTRYPOINT` and `CMD` (OCI: `Entrypoint`/`Cmd`)
+
+Ignored by bootc.
+
+It's recommended for bootc containers to set `CMD /sbin/init`; but this is not required.
+
+The booted host system will launch from the bootloader, to the kernel+initramfs and
+real root however it is "physically" configured inside the image.  Typically
+today this is using [systemd](https://systemd.io/) in both the initramfs
+and at runtime; but this is up to how you build the image.
+
+### `ENV` (OCI: `Env`)
+
+Ignored by bootc; to configure the global system environment you can
+change the systemd configuration.  (Though this is generally not a good idea;
+instead it's usually better to change the environment of individual services)
+
+### `EXPOSE` (OCI: `exposedPorts`)
+
+Ignored by bootc; it is agnostic to how the system firewall and network
+function at runtime.
+
+### `USER` (OCI: `User`)
+
+Ignored by bootc; typically you should configure individual services inside
+the bootc container to run as unprivileged users instead.