Add missing dependencies for install in fresh ubuntu

[jtylka]

hi, when I was trying to install osslsigncode into a fresh ubuntu docker image, I was getting CMake errors complaining about not finding ZLib and Python3, so I suggest adding those to the apt install command in the instructions.

FWIW, here is the dockerfile I made:

FROM ubuntu:22.04

RUN apt-get update
RUN apt-get install -y wget

WORKDIR /app
RUN wget https://github.com/mtrojnar/osslsigncode/archive/refs/tags/2.7.tar.gz
RUN tar -xf 2.7.tar.gz

RUN apt update && apt -y install cmake libssl-dev libcurl4-openssl-dev
RUN apt -y install zlib1g-dev python3

WORKDIR /app/osslsigncode-2.7
RUN mkdir build && cd build && cmake -S ..

WORKDIR /app/osslsigncode-2.7/build
RUN cmake --build .
# RUN ctest -C Release
RUN cmake --install .
RUN cmake --build . --target package_source

WORKDIR /app