@Override
        protected Long withPrefix(String prefix, ConfigKey key, Properties properties) {
            String prop = properties.getProperty(key.asPropertyKey(prefix));
            return prop == null ? null : Long.valueOf(prop);
        }
Data type-specific extractor to read value from properties.

@param prefix
@param key
@param properties
@return
src/main/java/io/fabric8/maven/docker/config/handler/property/ValueProvider.java