dolos: improve Dockerfile and extract build script from it

Refactoring and improving the Dockerfile used to build firmware.

BUG=b:373935038
TEST=create image and build firmware

Change-Id: I4d5c777171452d3883e015480c9fdf27e52f8047
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dolos/+/6089358
Reviewed-by: Keith Haddow <haddowk@chromium.org>
Tested-by: Michał Headcrab Barnaś <barnas@google.com>
Commit-Queue: Michał Headcrab Barnaś <barnas@google.com>