diff --git a/docker/.env b/docker/.env
index e704cbd2..d89c1080 100644
--- a/docker/.env
+++ b/docker/.env
@@ -1,3 +1,5 @@
+# Variables for compose.yml files. Not available in the app.
+
 STACK_NAME=app
 
 # Development
diff --git a/docker/dev/.env b/docker/dev/.env
index 8e5aaf70..6cb1d7dc 100644
--- a/docker/dev/.env
+++ b/docker/dev/.env
@@ -1,3 +1,6 @@
+# App dev environment.
+# Note that variables are available in the app only, not in compose.yml.
+
 APP_ENV=dev
 APP_DEBUG=true
 SERVER_NAME=:80
diff --git a/docker/prod/.env b/docker/prod/.env
index 78648a87..d5ca79d4 100644
--- a/docker/prod/.env
+++ b/docker/prod/.env
@@ -1,3 +1,6 @@
+# App prod environment.
+# Note that variables are available in the app only, not in compose.yml.
+
 APP_ENV=prod
 APP_DEBUG=false
 SERVER_NAME=:80
diff --git a/docker/test/.env b/docker/test/.env
index 7e1dd8ae..3dcbdaac 100644
--- a/docker/test/.env
+++ b/docker/test/.env
@@ -1,3 +1,6 @@
+# App test environment.
+# Note that variables are available in the app only, not in compose.yml.
+
 APP_ENV=test
 APP_DEBUG=false
 APP_C3=true