config: Explicit container namespace for uid, gid, and additionalGids

In the degenerate case where the container does not create a user
namespace, the "container namespace" distinction is unimportant, but
the phrasing is still accurate (the container and runtime namespaces
are the same).

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

config.md

@@ -112,9 +112,10 @@ The user for the process is a platform-specific structure that allows specific c
 
 For Linux-based systems the user structure has the following fields:
 
-* **`uid`** (int, required) specifies the user id.
-* **`gid`** (int, required) specifies the group id.
-* **`additionalGids`** (array of ints, optional) specifies additional group ids to be added to the process.
+* **`uid`** (int, required) specifies the user ID in the [container namespace][container-namespace].
+  For example, if a user has UID 1000 in their current namespace, and [starts][start] a container with a new [user namespace][namespaces] [mapping][user-namespace-mappings] the [runtime-namespace][] UID 1000 to the [container-namespace][] UID 0, they should set `uid` to 0 to have the process execute as 0 in container namespace (which will be 1000 in the runtime namespace).
+* **`gid`** (int, required) specifies the group ID in the [container namespace][container-namespace].
+* **`additionalGids`** (array of ints, optional) specifies additional group IDs (in the [container namespace][container-namespace]) to be added to the process.
 
 _Note: symbolic name for uid and gid, such as uname and gname respectively, are left to upper levels to derive (i.e. `/etc/passwd` parsing, NSS, etc)_
 
@@ -637,5 +638,10 @@ Here is a full example `config.json` for reference.
 }
 ```
 
+[container-namespace]: glossary.md#container-namespace
+[namespaces]: config-linux.md#namespaces
+[runtime-namespace]: glossary.md#runtime-namespace
+[start]: runtime.md#start
+[user-namespace-mappings]: config-linux#user-namespace-mappings
 
 [uts-namespace]: http://man7.org/linux/man-pages/man7/namespaces.7.html