Hey @Bruce27 
, I did get it to work.
I have broken the Nginx installation first though 
so if you want to try it try it on staging server if you have one.
I’ve installed the extras via apt, Nginx broke, I’ve then used the solution I found here and once I used nginx -V again I found the Lua module to be available.
--http-client-body-temp-path=/var/lib/nginx/body --http-fastcgi-temp-path=/var/lib/nginx/fastcgi --http-proxy-temp-path=/var/lib/nginx/proxy --http-scgi-temp-path=/var/lib/nginx/scgi --http-uwsgi-temp-path=/var/lib/nginx/uwsgi --with-debug --with-compat --with-pcre-jit --with-http_ssl_module --with-http_stub_status_module --with-http_realip_module --with-http_auth_request_module --with-http_v2_module --with-http_dav_module --with-http_slice_module --with-threads --with-http_addition_module --with-http_flv_module --with-http_geoip_module=dynamic --with-http_gunzip_module --with-http_gzip_static_module --with-http_image_filter_module=dynamic --with-http_mp4_module --with-http_perl_module=dynamic --with-http_random_index_module --with-http_secure_link_module --with-http_sub_module --with-http_xslt_module=dynamic --with-mail=dynamic --with-mail_ssl_module --with-stream=dynamic --with-stream_ssl_module --with-stream_ssl_preread_module --add-dynamic-module=/build/nginx-qv2Nx5/nginx-1.18.0/debian/modules/http-headers-more-filter --add-dynamic-module=/build/nginx-qv2Nx5/nginx-1.18.0/debian/modules/http-auth-pam --add-dynamic-module=/build/nginx-qv2Nx5/nginx-1.18.0/debian/modules/http-cache-purge --add-dynamic-module=/build/nginx-qv2Nx5/nginx-1.18.0/debian/modules/http-dav-ext --add-dynamic-module=/build/nginx-qv2Nx5/nginx-1.18.0/debian/modules/http-ndk --add-dynamic-module=/build/nginx-qv2Nx5/nginx-1.18.0/debian/modules/http-echo --add-dynamic-module=/build/nginx-qv2Nx5/nginx-1.18.0/debian/modules/http-fancyindex --add-dynamic-module=/build/nginx-qv2Nx5/nginx-1.18.0/debian/modules/nchan --add-dynamic-module=/build/nginx-qv2Nx5/nginx-1.18.0/debian/modules/http-lua --add-dynamic-module=/build/nginx-qv2Nx5/nginx-1.18.0/debian/modules/rtmp --add-dynamic-module=/build/nginx-qv2Nx5/nginx-1.18.0/debian/modules/http-uploadprogress --add-dynamic-module=/build/nginx-qv2Nx5/nginx-1.18.0/debian/modules/http-upstream-fair --add-dynamic-module=/build/nginx-qv2Nx5/nginx-1.18.0/debian/modules/http-subs-filter --add-dynamic-module=/build/nginx-qv2Nx5/nginx-1.18.0/debian/modules/http-geoip2
Once Lua was available I was able to create my MUplugin so that fastcgi cache can be cleared within the WordPress admin area