@VisibleForTesting
    Checksum hashCopiedFiles() {
        Checksum checksum = new Adler32();
        Stream
            .of(copyToFileContainerPathMap, copyToTransferableContainerPathMap)
            .flatMap(it -> it.entrySet().stream())
            .sorted(Entry.comparingByValue())
            .forEach(entry -> {
                byte[] pathBytes = entry.getValue().getBytes();
                // Add path to the hash
                checksum.update(pathBytes, 0, pathBytes.length);

                entry.getKey().updateChecksum(checksum);
            });
        return checksum;
    }
Starts the container using docker, pulling an image if necessary.
core/src/main/java/org/testcontainers/containers/GenericContainer.java