Docker: BEHIND_HTTPS and BEHIND_PROXY

Fix #164

bsmain/__init__.py

@@ -1,3 +1,4 @@
+import os
 import urllib.parse
 
 from django.core.checks import Warning, register
@@ -58,3 +59,20 @@ def check_base_url_is_url(app_configs, **kwargs):
         )]
 
     return []
+
+
+@register("bsmain")
+def check_proxy_env_vars_consistency(app_configs, **kwargs):
+    # in this check we straight-up check the os.environ: we can't rely on settings.BEHIND_HTTPS_PROXY to have been set
+    # since it's Docker-only.
+
+    if (
+        os.getenv("BEHIND_HTTPS_PROXY", "False").lower() in ("true", "1", "yes") and
+        os.getenv("BEHIND_PLAIN_HTTP_PROXY", "False").lower() in ("true", "1", "yes")
+            ):
+        return [Warning(
+            "BEHIND_HTTPS_PROXY and BEHIND_PLAIN_HTTP_PROXY are mutually exclusive.",
+            id="bsmain.W004",
+        )]
+
+    return []