remove hardcoded ldap libdir in Dockerfile

[BeckerFelix]

I found this repo to be regularly updated (thank you!) and thus I am using it to host a bookstack container on raspberry pi. For this purpose, I am rebuilding the image on the device with the Dockerfile provided. It works flawless despite one line:

docker-bookstack/Dockerfile

Line 9 in 2ccba9a

&& docker-php-ext-configure ldap --with-libdir=lib/x86_64-linux-gnu/ \

Obviously, the path lib/x86_64-linux-gnu does not exist on arm architecture. Removing the argument results in a successful build both on arm as well as amd64 systems.
As far as I see, the argument can be omitted to gain compatibility for arm. Let me know if I am missing anything.

[solidnerd]

Hey @MilkWithCoffee,

thanks for your pull request. I can't see any problems with this. Also all the checks have passed through the ci so we can integrate.

I'll merge it and it will be in the next release.