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
             )
         }
 

Tests/CLITests/Subcommands/Run/TestCLIRunCommand.swift

@@ -577,6 +577,68 @@ class TestCLIRunCommand: CLITest {
         }
     }
 
+    @Test func testRunCommandEnvFileFromNamedPipe() throws {
+        do {
+            let name = getTestName()
+            let pipePath = FileManager.default.temporaryDirectory.appendingPathComponent("envfile-pipe\(UUID().uuidString)")
+
+            // create pipe
+            let result = mkfifo(pipePath.path(), 0o600)
+            guard result == 0 else {
+                Issue.record("failed to create named pipe: \(String(cString: strerror(errno)))")
+                return
+            }
+
+            defer {
+                try? FileManager.default.removeItem(at: pipePath)
+            }
+
+            let content = """
+                FOO=bar
+                BAR=baz
+                """
+
+            let group = DispatchGroup()
+
+            group.enter()
+            DispatchQueue.global().async {
+                do {
+                    let handle = try FileHandle(forWritingTo: pipePath)
+                    try handle.write(contentsOf: Data(content.utf8))
+                    try handle.close()
+                } catch {
+                    Issue.record(error)
+                    return
+                }
+
+                group.leave()
+            }
+
+            try doLongRun(name: name, args: ["--env-file", pipePath.path()])
+            defer {
+                try? doStop(name: name)
+            }
+
+            group.wait()
+
+            let inspectResult = try inspectContainer(name)
+            let expected = [
+                "FOO=bar",
+                "BAR=baz",
+            ]
+
+            for item in expected {
+                #expect(
+                    inspectResult.configuration.initProcess.environment.contains(item),
+                    "expected environment variable \(item) not found"
+                )
+            }
+            try doStop(name: name)
+        } catch {
+            Issue.record(error)
+        }
+    }
+
     func getDefaultDomain() throws -> String? {
         let (_, output, err, status) = try run(arguments: ["system", "property", "get", "dns.domain"])
         try #require(status == 0, "default DNS domain retrieval returned status \(status): \(err)")

Tests/ContainerClientTests/ParserTest.swift

@@ -493,10 +493,12 @@ struct ParserTest {
         #expect {
             _ = try Parser.envFile(path: "/nonexistent/foo_bar_baz")
         } throws: { error in
-            guard let error = error as? ContainerizationError else {
+            guard let error = error as? ContainerizationError,
+                let cause = error.cause
+            else {
                 return false
             }
-            return error.description.contains("not found")
+            return String(describing: cause).contains("No such file or directory")
         }
     }
 
@@ -579,6 +581,41 @@ struct ParserTest {
         }
     }
 
+    @Test
+    func testParseEnvFileFromNamedPipe() throws {
+        let pipePath = FileManager.default.temporaryDirectory
+            .appendingPathComponent("envfile-pipe-\(UUID().uuidString)")
+
+        // Create a named pipe (FIFO)
+        let result = mkfifo(pipePath.path, 0o600)
+        guard result == 0 else {
+            throw POSIXError(POSIXErrorCode(rawValue: errno) ?? .EPERM)
+        }
+        defer { try? FileManager.default.removeItem(at: pipePath) }
+
+        let group = DispatchGroup()
+
+        group.enter()
+        DispatchQueue.global().async {
+            do {
+                let handle = try FileHandle(forWritingTo: pipePath)
+                try handle.write(contentsOf: "SECRET_KEY=value123\n".data(using: .utf8)!)
+                try handle.close()
+            } catch {
+                Issue.record(error)
+            }
+            group.leave()
+        }
+
+        // Read from pipe (blocks until writer connects)
+        let lines = try Parser.envFile(path: pipePath.path)
+
+        // Wait for write to complete
+        group.wait()
+
+        #expect(lines == ["SECRET_KEY=value123"])
+    }
+
     // MARK: Network Parser Tests
 
     @Test