doc/ephemeral: Describe how to detect the environment

[cgwalters]

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

[gemini-code-assist]

Code Review

This pull request adds documentation about detecting the bcvk ephemeral environment, which is useful for adapting systemd units. It also documents the --env option for bcvk ephemeral run and bcvk ephemeral run-ssh. The changes are clear and helpful. I've suggested a few minor wording improvements in ephemeral-run.md to enhance clarity and fix some grammatical points.

[gemini-code-assist]

This new section is very helpful! I have a few suggestions to improve grammar, spelling, and clarity:

• On line 27, "there's" should be "there are" since it refers to multiple items.
• On line 35, "adaption" should be "adaptation".
• The final sentence on lines 40-41 is a bit tentative ("should work reliably in the future"). If this is the recommended method now, we could phrase it more confidently and explain its purpose.

Here's a suggested revision for the whole section that incorporates these points:

Suggested change

	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.
	Conceptually now with `bcvk ephemeral`, there are *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 adaptation 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.