Allow dashes in environment variable names

See http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html
Environment variable names used by the utilities in the Shell and
Utilities volume of POSIX.1-2008 consist solely of uppercase letters,
digits, and the <underscore> ( '_' ) from the characters defined in
Portable Character Set and do not begin with a digit. Other characters may
be permitted by an implementation; applications shall tolerate the
presence of such names.

Signed-off-by: Joffrey F <joffrey@docker.com>

compose/config/fields_schema.json

@@ -40,7 +40,7 @@
             {
               "type": "object",
               "patternProperties": {
-                "^[^-]+$": {
+                ".+": {
                   "type": ["string", "number", "boolean", "null"],
                   "format": "environment"
                 }