Provide a way to inject environment variables and/or scripts into the Docker container used for Linux builds

[freakboy3742]

AppImage builds require that most dependencies are build from source. However, in order to build Cryptography, it is necessary to install a Rust development toolchain. This can't be done with a simple system_requires declaration.

If you revert to an older version of Cryptography (e.g., 3.4.8) where the Rust build was optional, you still need to declare an environment variable (CRYPTOGRAPHY_DONT_BUILD_RUST=1) to disable the Rust build.

Describe the solution you'd like

Functionally - it should be possible to install Cryptography - either a recent release, or a historical pre-rust release.

This likely requires the ability to inject full build steps into the Docker image used to build AppImages.
e.g.,

extra_docker_steps = [
   "ENV CRYPTOGRAPHY_DONT_BUILD_RUST=1",
   "RUN echo 'hello world'",
]

or

extra_docker_steps = [
   "RUN curl https://sh.rustup.rs -sSf | sh",
]

These declarations would be injected into the Dockerfile - probably towards the end of the Dockerfile, after the installation of system packages.

Describe alternatives you've considered

We could make a special case of Rust, and add a enable-rust = true option; however, the more flexible option will be to provide complete environment flexibility.

extra_docker_steps could also be a single multiline string, rather than a list of strings.