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

msmtp.conf.tmpl

@@ -1,19 +1,19 @@
 account default
-host {{ env "SMTP_HOST" }}
-from {{ env "MAIL_FROM" }}
-user {{ or (env "SMTP_USER") (env "MAIL_FROM") }}
-port {{ env "SMTP_PORT" }}
+host {{ getenv "SMTP_HOST" }}
+from {{ getenv "MAIL_FROM" }}
+user {{ or (getenv "SMTP_USER") (getenv "MAIL_FROM") }}
+port {{ getenv "SMTP_PORT" }}
 
-{{ if eq (env "SMTP_OVERRIDE_FROM") "on" }}
+{{ if eq (getenv "SMTP_OVERRIDE_FROM") "on" }}
 set_from_header on
 {{ end }}
 
-{{ if eq (env "SMTP_AUTH") "on" }}
-auth {{ env "SMTP_AUTH" }}
+{{ if eq (getenv "SMTP_AUTH") "on" }}
+auth {{ getenv "SMTP_AUTH" }}
 passwordeval "cat /run/secrets/smtp_password"
 {{ end }}
 
-{{ if eq (env "SMTP_TLS") "on" }}
-tls {{ env "SMTP_TLS" }}
+{{ if eq (getenv "SMTP_TLS") "on" }}
+tls {{ getenv "SMTP_TLS" }}
 tls_trust_file /etc/ssl/certs/ca-certificates.crt
 {{ end }}