Container engines with operating-system virtualization have been widely used and now offer extensions to replace core functionalities that are derived from the host kernel. Because such extensions with an alternate kernel, which is often implemented in a library operating system (libOS), can be designed to have free choice, developers are tempted to take a clean-slate approach, i.e., implement the kernels from scratch. However, this design decision makes it difficult to cover broad features of the original Linux kernel, and some application programs may not work on such kernels. Precise emulation of the huge codebase and rich feature set of the Linux kernel is not easily possible. In this paper, we have tried to improve the level of compatibility in a libOS by using the source code of the Linux kernel as the container kernel. We present µKontainer, an alternate container kernel based on a libOS by extending the existing open-source software, Linux Kernel Library, while preserving the lightweight property of conventional containers. We have studied the level of compatibility with the conformance tests of network protocol implementation of nine different libOSs, and µKontainer performs identically like the Linux kernel. The network-related benchmark shows mostly comparable results with a conventional container and a native Linux host; in the best case, the goodput of the short-sized packet is up to 84% faster than that of a native Linux host. This paper sheds light on the design space of the libOS when we introduced the extended container kernel.