config: Adjust process.env to drop name charset limitations

More closely following POSIX's phrasing:

  These strings have the form name=value; names shall not contain the
  character '='. For values to be portable across systems conforming
  to IEEE Std 1003.1-2001, the value shall be composed of characters
  from the portable character set (except NUL and as indicated below).

The uppercase letter, digit, underscore restriction is just for
"variables used by the utilities in the Shell and Utilities volume of
IEEE Std 1003.1-2001".

I've left off the charset requirement; we're punting to POSIX and
copying that over and then linking to POSIX didn't seem like much
gain.  Just point people at POSIX and let them read about the charset
suggestions there.

The MUST NOT bit isn't particularly useful, and just rules out the
'=value' case (which would have an empty name).  A rephrasing of the
requirement (which I didn't use, because it seemed too different from
the POSIX wording) would be:

  These strings MUST contain the character '=' and MUST NOT have '='
  as the first character.

Also link specifically to chapter 8 section 1 (instead of just chapter
8).

The "platform-appropriate way" wording comes from Rob Dolin [1], and
gives space for non-POSIX systems to define their own environment
variable format (and hopefully include links to their specifications
in the spec as well).

[1]: http://ircbot.wl.linuxfoundation.org/meetings/opencontainers/2016/opencontainers.2016-05-18-17.01.log.html#l-54

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

Author: wking

config.md

@@ -88,7 +88,9 @@ See links for details about [mountvol](http://ss64.com/nt/mountvol.html) and [Se
 
 * **`terminal`** (bool, optional) specifies whether you want a terminal attached to that process. Defaults to false.
 * **`cwd`** (string, required) is the working directory that will be set for the executable. This value MUST be an absolute path.
-* **`env`** (array of strings, optional) contains a list of variables that will be set in the process's environment prior to execution. Elements in the array are specified as Strings in the form "KEY=value". The left hand side MUST consist solely of letters, digits, and underscores `_` as outlined in [IEEE Std 1003.1-2001](http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap08.html).
+* **`env`** (array of strings, optional) contains a list of variables that will be set in the process's environment prior to execution.
+  Elements in the array are specified in a platform-appropriate way.
+  On POSIX platforms the strings MUST have the form *name=value*, where *name* MUST NOT contain the character `=`, as outlined in [IEEE Std 1003.1-2001][ieee-1003.1-2001-xbd-c8.1].
 * **`args`** (array of strings, required) executable to launch and any flags as an array. The executable is the first element and MUST be available at the given path inside of the rootfs. If the executable path is not an absolute path then the search $PATH is interpreted to find the executable.
 
 For Linux-based systems the process structure supports the following process specific fields:
@@ -662,5 +664,6 @@ Here is a full example `config.json` for reference.
 }
 ```
 
+[ieee-1003.1-2001-xbd-c8.1]: http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap08.html#tag_08_01
 [runtime-namespace]: glossary.md#runtime-namespace
 [uts-namespace]: http://man7.org/linux/man-pages/man7/namespaces.7.html