pin to bookworm to fix part of the build

[ericcj]

Since debian 13 Trixie became stable on 2025-08-09, builds were failing with "Unable to locate package libpcre3" so this pins it back to bookworm until we decide to update dependencies.

However builds still fail on dlib==19.24.4:

108.7 [100%] Linking CXX shared module /tmp/pip-install-d_g9vonj/dlib_3290ea725ee64fac880094072fa2ce43/build/lib.linux-x86_64-cpython-311/_dlib_pybind11.cpython-311-x86_64-linux-gnu.so
108.7 In member function ‘allocate_array’,
108.7 inlined from ‘set_max_size.constprop’ at /tmp/pip-install-d_g9vonj/dlib_3290ea725ee64fac880094072fa2ce43/dlib/../dlib/matrix/../array/array_kernel.h:438:59:
108.7 /tmp/pip-install-d_g9vonj/dlib_3290ea725ee64fac880094072fa2ce43/dlib/../dlib/memory_manager_stateless/memory_manager_stateless_kernel_1.h:54:24: warning: argument 1 value ‘18446744073709551615’ exceeds maximum object size 9223372036854775807 [-Walloc-size-larger-than=]
108.7 54 | return new T[size];
108.7 | ^
108.7 /usr/include/c++/12/new: In member function ‘set_max_size.constprop’:
108.7 /usr/include/c++/12/new:128:26: note: in a call to allocation function ‘operator new []’ declared here
108.7 128 | _GLIBCXX_NODISCARD void* operator new _GLIBCXX_THROW (std::bad_alloc)
108.7 | ^

But it sounds like maybe this is a pre-existing cross-compilation issue have we been doing these builds from a linux host?
davisking/dlib#3038 (comment)

[ericcj]

i confirmed the build works fine with this branch in git@deploy.pushd.com so gonna merge as bookworm must have been how we last built imgproxy even though others saw issues with it imgproxy#1330 and upstream has switched to ubuntu so whenever we merge there latest it'll conflict https://github.com/imgproxy/imgproxy/blob/master/docker/Dockerfile