protected synchronized Date getBuildTimestamp() throws IOException {
        Date now = (Date) getPluginContext().get(CONTEXT_KEY_BUILD_TIMESTAMP);
        if (now == null) {
            now = getReferenceDate();
            getPluginContext().put(CONTEXT_KEY_BUILD_TIMESTAMP,now);
        }
        return now;
    }
Get the current build timestamp. this has either already been created by a previous
call or a new current date is created
@return timestamp to use
src/main/java/io/fabric8/maven/docker/AbstractDockerMojo.java