Add support for reading env from named pipes (#974)

This is a fix for
[issue#956](https://github.com/apple/container/issues/956)

`FileManager.default.contents(atPath:)` returns `nil` for named pipes
(FIFOs)
and process substitutions like `/dev/fd/XX` because:
1. It expects regular files with a known size
2. Named pipes are stream-based and block until data arrives

## Solution
Use `FileHandle(forReadingFrom:)` instead, which:
- Properly handles blocking I/O
- Works with named pipes, process substitutions, and regular files
(mentioned in the
[doc](https://developer.apple.com/documentation/foundation/filehandle))

Co-authored-by: Bortniak Volodymyr <Bortnyak@users.noreply.github.com>

Sources/ContainerClient/Parser.swift

@@ -110,17 +110,19 @@ public struct Parser {
         // This is a somewhat faithful Go->Swift port of Moby's envfile
         // parsing in the cli:
         // https://github.com/docker/cli/blob/f5a7a3c72eb35fc5ba9c4d65a2a0e2e1bd216bf2/pkg/kvfile/kvfile.go#L81
-        guard FileManager.default.fileExists(atPath: path) else {
-            throw ContainerizationError(
-                .notFound,
-                message: "envfile at \(path) not found"
-            )
-        }
 
-        guard let data = FileManager.default.contents(atPath: path) else {
+        let data: Data
+        do {
+            // Use FileHandle to support named pipes (FIFOs) and process substitutions
+            // like --env-file <(echo "KEY=value")
+            let fileHandle = try FileHandle(forReadingFrom: URL(fileURLWithPath: path))
+            defer { try? fileHandle.close() }
+            data = try fileHandle.readToEnd() ?? Data()
+        } catch {
             throw ContainerizationError(
                 .invalidArgument,
-                message: "failed to read envfile at \(path)"
+                message: "failed to read envfile at \(path)",
+                cause: error
             )
         }