Improve docker build time when cache misses

[felipecrs]

After #228, build started taking more than 3 hours instead of 30 minutes. Build now takes 1 hour and 30 minutes.

The reason why I refactored the docker build was to allow users to have a single tag to pull from regardless of the architecture, which is prettier and simpler.

I tried to improve with #236, but it apparently caused #238.

For now I will revert #236, so cached builds will still be quick, but non-cached builds will still take very long.

There is one idea on docker/build-push-action#846 on how to probably fix this, but I don't like it much.

I will leave this issue open in case someone has some idea, or even myself.

[felipecrs]

One idea to improve, for example, is to use cross-platform build of Asterisk source code.

I don't know if its Makefile supports it, but if we can build Asterisk for arm64 on amd64 without having to use binfmt/qemu for that, it would speed up the build by a lot.

[felipecrs]

I did some research and cross compilation of Asterisk is possible.

[felipecrs]

https://wiki.beyondlogic.org/index.php?title=Cross_Compiling_Asterisk_IPPBX_for_ARM