Revert #66 build libvips twice

[CGDogan]

OpenSlide now reports it on time if it found an incompatible dicom, so libvips will of course try a different reader. So there's no benefit left in building it twice

[birm]

I'm working on testing on a computer which isn't my work mac, but on the M1, I got " > [loader 9/21] RUN python3 compile_bfbridge.py:
#0 0.355 Traceback (most recent call last):
#0 0.356 File "/root/src/BFBridge/python/compile_bfbridge.py", line 1, in
#0 0.357 from cffi import FFI
#0 0.358 ModuleNotFoundError: No module named 'cffi'"

I think we need to install the requirements.txt in /root/src/BFBridge/python

I'll comment if I get a different result on a x86 host.

[birm]

I got the same result:: Step 9/25 : RUN python3 compile_bfbridge.py
---> Running in 67c7bd0be163
Traceback (most recent call last):
File "/root/src/BFBridge/python/compile_bfbridge.py", line 1, in
from cffi import FFI
ModuleNotFoundError: No module named 'cffi'"

[birm]

Awesome, thanks for the fixes!