doc/ephemeral: Describe how to detect the environment

`bcvk to-disk quay.io/fedora/fedora-coreos` is broken
because it has a generator that fails in this environment.

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

Author: cgwalters

docs/src/ephemeral-run.md

@@ -22,6 +22,24 @@ bcvk ephemeral run-ssh quay.io/fedora/fedora-bootc:42
 bcvk ephemeral run --memory 4096 --cpus 4 --name myvm quay.io/fedora/fedora-bootc:42
 ```
 
+## Detecting an ephemeral environment
+
+Conceptually now with `bcvk ephemeral`, there's *four* different ways to run
+a bootc container:
+
+- `podman|docker run <image> bash` - directly run a shell (or other process) the container without systemd. Uses the host kernel, not kernel in the container.
+- `podman|docker run <image>` - by default runs systemd. See also <https://docs.fedoraproject.org/en-US/bootc/provisioning-container/>. Uses the host kernel, not kernel in the container.
+- `bootc install` - Run directly on metal or a virtualized environment. Uses the kernel in the container.
+- `bcvk ephemeral` - Run as a virtual machine, but *not* a true "bootc install". Uses the kernel in the container.
+
+Some systemd units may need adaption to work in all of these modes. For example, if you have a systemd generator
+which synthesizes mount units for expected partitions, it can use `ConditionVirtualization=!container` to skip
+those in the first two cases (ensuring it still runs after a `bootc install`), but that won't be skipped in `bcvk ephemeral`
+even though there won't be any block devices (by default).
+
+At the current time there is not a dedicated way to detect `bcvk ephemeral`, but `ConditionKernelCommandLine=!rootfstype=virtiofs`
+should work reliably in the future.
+
 ## Use Cases
 
 - Quick testing of bootc images