Reduce amount layers of the docker image

[felipecrs]

This reduces the amount of layers in the image from 13 to 5. Less layers is a recommended practice for Docker images. It should make pulling the image faster, and perhaps speed up the container startup time.

This also reduces the size of the image from 1.53GB to 1.52GB, which isn't noticeable, I know.

It as well makes the build process faster, as it parallelizes several other steps.

As well as some other improvements, like:

• Add go2rtc and nginx bin folders to PATH
• Refactor s6-related scripts, so they are more readable
• Convert long dockerfile RUN instructions to shell scripts, so they are more readable and can be easily linted with shellcheck

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[blakeblackshear]

There are some conflicts from your other PR

[felipecrs]

There are some conflicts from your other PR

Thanks. Fixed now.