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

htaccess.tmpl

@@ -3,7 +3,7 @@
     Require all denied
 </FilesMatch>
 
-{{ if eq (env "MULTISITE") "" -}}
+{{ if eq (getenv "MULTISITE") "" -}}
 # BEGIN WordPress
 
 RewriteEngine On
@@ -17,7 +17,7 @@ RewriteRule . /index.php [L]
 # END WordPress
 {{- end -}}
 
-{{- if eq (env "MULTISITE") "subfolder" -}}
+{{- if eq (getenv "MULTISITE") "subfolder" -}}
 # BEGIN WordPress Multisite
 # Using subfolder network type: https://wordpress.org/documentation/article/htaccess/#multisite
 
@@ -39,7 +39,7 @@ RewriteRule . index.php [L]
 # END WordPress Multisite
 {{- end -}}
 
-{{- if eq (env "MULTISITE") "subdomain" -}}
+{{- if eq (getenv "MULTISITE") "subdomain" -}}
 # BEGIN WordPress Multisite
 # Using subdomain network type: https://wordpress.org/documentation/article/htaccess/#multisite