Add an error message for dockerfile >= 16KB until apple#735 is resolved (apple#1634)

Closes apple#1633. We have a known
issue apple#735 where Dockerfiles
over the size 16kb will fail to build due to "Transport became inactive"
or "Stream unexpectedly closed" errors. While we wait for a fix for
apple#735, this PR adds an error
message if a user tries to build an image using a dockerfile >= 16kb.

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

Author: katiewasnothere

Sources/ContainerCommands/BuildCommand.swift

@@ -251,6 +251,20 @@ extension Application {
                     ignoreFileData = try? Data(contentsOf: ignoreFileURL)
                 }
 
+                // BUG: See https://github.com/apple/container/issues/735.
+                // Reject dockerfiles larger than 16kb before attempting to build.
+                // TODO: Remove when #735 was been resolved.
+                let maxDockerfileSize = 16 * 1024  // 16 KiB
+                guard buildFileData.count < maxDockerfileSize else {
+                    throw ContainerizationError(
+                        .invalidArgument,
+                        message: """
+                            Dockerfile size (\(buildFileData.count) bytes) exceeds the maximum allowed size of \(maxDockerfileSize) bytes. \
+                            See https://github.com/apple/container/issues/735.
+                            """
+                    )
+                }
+
                 let secretsData: [String: Data] = try self.secrets.mapValues { secret in
                     switch secret {
                     case .data(let data):