Fix lint issues and improve test suite resilience

- compose.matrix.yml: add trailing newline
- compose.ftp-222*.yml: fix YAML indentation (8->6 spaces)
- entrypoint.sh.tmpl: fix SC2198 ([ -n "$@" ] -> [ $# -gt 0 ])
- All .tmpl files: migrate from {{ env }} to {{ getenv }} for gomplate v5 compat
- uploads.ini.tmpl: add {{- / -}} whitespace trimming, fix double-space typo
- tests/run.sh: only run ShellCheck on *.sh.tmpl files
- tests/test_shell.sh: gracefully skip if shellcheck not installed
- tests/test_templates.sh: remove dead render() function,
  gracefully skip if gomplate not found, use set -a/. for env sourcing
- tests/test_compose_config.sh: validate override files combined
  with compose.yml, skip partial snippets needing more context
- README.md: add test instructions with brew install

README.md

@@ -74,6 +74,31 @@ Below are the instructions for the local relay.
    `$DOMAIN` or in its `$EXTRA_SENDER_DOMAINS`
 3. `abra app deploy <app-name>`
 
+## Tests
+
+Run the full test suite:
+
+```sh
+bash tests/run.sh
+```
+
+### Prerequisites
+
+The test suite uses several tools. Install them with:
+
+```sh
+brew install shellcheck gomplate
+```
+
+Or on Debian/Ubuntu:
+
+```sh
+sudo apt-get install shellcheck
+# gomplate: https://github.com/hairyhenderson/gomplate/releases
+```
+
+Some tests skip gracefully if their dependencies are missing.
+
 [abra]: https://git.autonomic.zone/autonomic-cooperative/abra
 [cc-traefik]: https://git.autonomic.zone/coop-cloud/traefik
 [cc-postfix-relay]: https://git.autonomic.zone/coop-cloud/traefik