Enumerate using relative paths to avoid mismatch with symlink resolution of special paths like /tmp (apple#613)

## Type of Change
- [x] Bug fix
- [ ] New feature  
- [ ] Breaking change
- [ ] Documentation update

## Motivation and Context
Fixes apple#588. This PR changes the
archiver compression file enumeration to use the
[enumerator(atPath:)](https://developer.apple.com/documentation/foundation/filemanager/enumerator(atpath:))
version. This version returns relative paths instead of full file paths
from the filesystem. /tmp is symlinked to /private/tmp and some swift
packages will handle that path differently. While a call to Foundation's
`URL.resolvingSymlinksInPath()` will return "/tmp", a call to
`FileManager.enumerator(at:)` will return "/private/tmp". This
difference causes a container image build to fail when the user is using
a path under /tmp or other special case paths as the context directory.

## Testing
- [x] Tested locally
- [x] Added/updated tests
- [ ] Added/updated docs

Signed-off-by: Kathryn Baldauf <k_baldauf@apple.com>

Author: katiewasnothere

Sources/ContainerClient/Archiver.swift

@@ -60,10 +60,7 @@ public final class Archiver: Sendable {
             try fileManager.createDirectory(at: directory, withIntermediateDirectories: true)
 
             guard
-                let enumerator = FileManager.default.enumerator(
-                    at: source,
-                    includingPropertiesForKeys: [.isDirectoryKey, .isRegularFileKey, .isSymbolicLinkKey]
-                )
+                let enumerator = FileManager.default.enumerator(atPath: source.path)
             else {
                 throw Error.fileDoesNotExist(source)
             }
@@ -74,7 +71,8 @@ public final class Archiver: Sendable {
                     entryInfo.append(info)
                 }
             } else {
-                while let url = enumerator.nextObject() as? URL {
+                while let relPath = enumerator.nextObject() as? String {
+                    let url = source.appending(path: relPath).standardizedFileURL
                     guard let info = closure(url) else {
                         continue
                     }

Tests/CLITests/Subcommands/Build/CLIBuilderTest.swift

@@ -94,6 +94,27 @@ extension TestCLIBuildBase {
             #expect(try self.inspectImage(newImageName) == newImageName, "expected to have successfully built \(newImageName)")
         }
 
+        @Test func testBuildAddFromSpecialDirs() throws {
+            let tempDir = URL(filePath: "/tmp/container/.clitests" + testUUID)
+            try! FileManager.default.createDirectory(at: tempDir, withIntermediateDirectories: true)
+
+            defer {
+                try! FileManager.default.removeItem(at: tempDir)
+            }
+
+            let dockerfile: String =
+                """
+                FROM scratch
+
+                ADD emptyFile /
+                """
+            let context: [FileSystemEntry] = [.file("emptyFile", content: .zeroFilled(size: 1))]
+            try createContext(tempDir: tempDir, dockerfile: dockerfile, context: context)
+            let imageName = "registry.local/scratch-add-special-dir:\(UUID().uuidString)"
+            try self.build(tag: imageName, tempDir: tempDir)
+            #expect(try self.inspectImage(imageName) == imageName, "expected to have successfully built \(imageName)")
+        }
+
         @Test func testBuildScratchAdd() throws {
             let tempDir: URL = try createTempDir()
             let dockerfile: String =