Separate stdout and stderr in runExec

[joshtrichards]

runExec currently merges stdout and stderr into the same PassThrough stream:

nextcloud-e2e-test-server/lib/docker.ts

Line 411 in 7802184

exec.modem.demuxStream(stream, dataStream, dataStream)

Ideally we separate them.

This would allow us to capture error output - e.g. from occ - separately when we care about it.

Then we could make some scenarios like the docker-modem workaround in runOcc more robust:

nextcloud-e2e-test-server/lib/docker.ts

Lines 259 to 262 in 7802184

	// Build app list
	const json = await runOcc(['app:list', '--output', 'json'], { container })
	// fix dockerode bug returning invalid leading characters
	const applist = JSON.parse(json.substring(json.indexOf('{')))

e.g.:

const { stdout, stderr } = await runOcc(['app:list', '--output', 'json'], { container })
if (stderr.trim()) {
  console.warn(`├─ occ stderr: ${stderr.trim()}`)
}
// Strip any residual binary header bytes from stdout only (docker-modem demuxing bug)
const cleaned = stdout.replace(/^[\x00-\x08\x0e-\x1f]+/, '')
const applist = JSON.parse(cleaned)

[joshtrichards]

Related upstream fix for the demux bug: apocas/docker-modem#195