capsh doesn't work with the container image which has no /bin/bash

[mhiramat]

Capsh is a great tool to drop capabilities for preventing jailbreak from chroot. However, it hardcodes /bin/bash to run(in chrooted rootfs), thus we can not run containers which don't have /bin/bash.

Maybe we have 3 options;

• If there is no /bin/bash but /bin/sh, add a wrapper shell script as /bin/bash. (which just exec /bin/sh with given parameters)
• If there is no /bin/sh, we just fail to run, and warn the reason.
• Fix capsh (to use /bin/sh or just directly run given command) or make another command.

[mhiramat]

This commit just avoid using capsh. I would like to try contribute capsh to support -x (exec) mode or make capexec to fix this issue.

[mhiramat]

See my libcap repository. I committed 2 patches which allow minc to execute given command directly from capsh.