config-loader: switch to using --config options to load in config

.changeset/dull-icons-share.md

@@ -0,0 +1,21 @@
+---
+'@backstage/backend-common': minor
+'@backstage/cli': minor
+'@backstage/config-loader': minor
+'example-backend': patch
+'@backstage/create-app': patch
+---
+
+**BREAKING CHANGE**
+
+The existing loading of additional config files like `app-config.development.yaml` using APP_ENV or NODE_ENV has been removed.
+Instead, the CLI and backend process now accept one or more `--config` flags to load config files.
+
+Without passing any flags, `app-config.yaml` and, if it exists, `app-config.local.yaml` will be loaded.
+If passing any `--config <path>` flags, only those files will be loaded, **NOT** the default `app-config.yaml` one.
+
+The old behaviour of for example `APP_ENV=development` can be replicated using the following flags:
+
+```bash
+--config ../../app-config.yaml --config ../../app-config.development.yaml
+```