Disallow booleans in environment

When users were putting true/false/yes/no in the environment key,
the YML parser was converting them into True/False, rather than leaving
them as a string.

This change will force people to put them in quotes, thus ensuring
that the value gets passed through as intended.

Signed-off-by: Mazz Mosley <mazz@houseofmnowster.com>

compose/config/fields_schema.json

@@ -36,7 +36,15 @@
 
         "environment": {
           "oneOf": [
-            {"type": "object"},
+            {
+              "type": "object",
+              "patternProperties": {
+                "^[a-zA-Z0-9_]+$": {
+                  "type": ["string", "number"]
+                }
+              },
+              "additionalProperties": false
+            },
             {"type": "array", "items": {"type": "string"}, "uniqueItems": true}
           ]
         },