This patch opts into the `__screnshots__` folder snapshot management
for docker.
With this patch, docker-originating snapshots will be stored in the
following folder:
```
{testDir}/__screenshots__/{projectName}/{testFilePath}/{snapshotName}
```
Where `{testFilePath}` is a test file path relative to `testDir`
Drive-by: introduce and document the `PLAYWRIGHT_DOCKER` environment
variable that enables docker integration.
