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

tests/test_shell.sh

@@ -4,6 +4,15 @@ set -euo pipefail
 pass=0
 fail=0
 
+# Skip if shellcheck is not installed
+if ! command -v shellcheck &>/dev/null; then
+    echo "=== ShellCheck ==="
+    echo "  SKIP  shellcheck not found"
+    echo "---"
+    echo "Passed: 0  Failed: 0"
+    exit 0
+fi
+
 # Allow overriding shellcheck options via env var (e.g. -s bash)
 EXTRA_SHELLCHECK_OPTS="${SHELLCHECK_OPTS:-}"