Customize broken since luarocks-admin not on path

[morvaiff]

customize/Dockerfile cannot be used to build custom Kong images with additional bundled plugins from a curated list, since packer.lua invokes luarocks-admin in a shell, but the tool is not available on the system path.

********************************************************************************
*                            Set up LuaRocks server                            *
********************************************************************************
[packer exec] luarocks-admin make_manifest /rocks-server
sh: 1: luarocks-admin: not found
/usr/local/openresty/luajit/bin/luajit: /packer.lua:249: 127
stack traceback:
        [C]: in function 'assert'
        /packer.lua:249: in function 'start_rocks_server'
        /packer.lua:285: in main chunk
        [C]: at 0x5620da6e5450
The command '/bin/sh -c /usr/local/openresty/luajit/bin/luajit /packer.lua -- "$INJECTED_PLUGINS"' returned a non-zero code: 1

I see this was addressed in 9cc3ca4 and faf4f06, but then removed in 9d58e41, together with some test cases.

Shouldn't the luarocks-admin symbolic link in /usr/local/bin be included after all (either as in the above commits, or in the build stage of customize/Dockerfile)? Or does the removal of tests indicate that creating custom images this way is no longer supported?