linux: relax filesystem requirements for container

change MUST to SHOULD so containers are not required to have all these
filesystems mounted.

Signed-off-by: Daniel Dao <[email protected]>

Author: dqminh

config-linux.md

@@ -8,7 +8,7 @@ The Linux container specification uses various kernel features like namespaces,
 The Linux ABI includes both syscalls and several special file paths.
 Applications expecting a Linux environment will very likely expect these file paths to be setup correctly.
 
-The following filesystems MUST be made available in each container's filesystem:
+The following filesystems SHOULD be made available in each container's filesystem:
 
 |   Path   |  Type  |
 | -------- | ------ |