CI observability enhancements. (#1193)

- Adds a a `--log-root` option to `swift system start`, propagating the
value as `CONTAINER_LOG_ROOT` to services for logging to files instead
of the OS log facility. This is not a "production" capability as it
neither merges nor rotates logs.
- Currently we don't collect logs on CI builds, and we don't have
permission to run the `log` command there. The PR adds `--log-root` to
the CI test phase, archives the results, and uploads the archive as an
artifact.
- Use FilePath from swift-system for the log root. Foundation URL is a
bit of a footgun for filesystem paths, so unless we identify a
showstopper, we should incrementally transition to this type everywhere
except where we really need network URLs.
- Output the hostname of the CI runner at the start of the test phase so
we can identify runner-specific issues where they exist.
- Fix formatting for log messages with multiple metadata items, and fix
unstructured messages on instances that weren't found using `grep -r
'log\.' Sources`.
- Adds command reference documentation for `--log-root`.

Sources/ContainerCommands/Image/ImageInspect.swift

@@ -63,9 +63,13 @@ extension Application {
             }
 
             if !allErrors.isEmpty {
-                let logger = Logger(label: "ImageInspect", factory: { _ in StderrLogHandler() })
                 for (name, error) in allErrors {
-                    logger.error("\(name): \(error.localizedDescription)")
+                    log.error(
+                        "image inspect failed",
+                        metadata: [
+                            "name": "\(name)",
+                            "error": "\(error.localizedDescription)",
+                        ])
                 }
 
                 throw InspectError(succeeded: succeededImages, failed: allErrors)