new: test: added a test to ensure ``$COMPOSE_YML_FILE`` is correctly accessible from actions.

test/action

@@ -81,6 +81,27 @@ expected="hello from foo: a b c -y -z j --help"
 }
 
 
+## -- direct charm access to \$COMPOSE_YML_FILE
+
+
+    cat <<'EOF2' > $test_tmpdir/www/actions/foo
+#!/bin/bash
+
+printf "COMPOSE_YML_FILE: '%s'" "\$COMPOSE_YML_FILE"
+EOF2
+
+cd "$test_tmpdir"
+out=\$("$tprog" foo web_site a b c -y -z j --help 2>&1 >/dev/null ) || exit 1
+expected="COMPOSE_YML_FILE: '$test_tmpdir/compose.yml'"
+
+[[ "\$out" == *"\$expected" ]] || {
+    echo "doesn't end with: \$expected" >&2
+    echo "\$out"
+    exit 1
+}
+
+
+
 EOF
 
     tear_test