private void addShell(StringBuilder b) {
        if (shell != null) {
            buildArgumentsAsJsonFormat(b, DockerFileKeyword.SHELL, true, shell);
        }
    }
Escape any slashes, quotes, and newlines int the value.  If any escaping occurred, quote the value.
@param key The key
@param value The value
@return Escaped and quoted key="value"
src/main/java/io/fabric8/maven/docker/assembly/DockerFileBuilder.java