@Override
            String reencodeRecording(@NonNull VncRecordingContainer container, @NonNull String source)
                throws IOException, InterruptedException {
                String newFileOutput = "/newScreen.flv";
                container.execInContainer("ffmpeg", "-i", source, "-vcodec", "libx264", newFileOutput);
                return newFileOutput;
            }
Create a sidekick container and attach it to another container. The VNC output of that container will be recorded.
core/src/main/java/org/testcontainers/containers/VncRecordingContainer.java