Update container image to build libs documentation.

[giomba]

I think that the title is self-explanatory.
I moved the set(Z80...) variable before the include of the library, because it is needed to build the docs.
This changes the previous behaviour (docs are always built, instead of only being built when building in debug mode, and the build output is very verbose), but this doesn't look like a big issues, since CMake is smart enough to only build it the first time.

[giuliof]

Is the documentation mandatory to build the emulator?

[giomba]

Is the documentation mandatory to build the emulator?

No, it is useful only for development.

[giuliof]

To avoid the documentation to be built by default, I suggest to not set this and to add in the documentation to use -DZ80_WITH_HTML_DOCUMENTATION=True in configuration to optionally build it.

[giomba]

I preferred not to have to remember which particular -D flag I had to pass to the build in order to build the documentation, and make it all automagic, but I can still see your point. Ok, I'll just remove it.