Split Docker toolchain stage and make entrypoint overridable

[nitroxit]

Summary

• split the slow-changing toolchain into a reusable Docker stage
• copy the current checkout into the final image
• replace the shell-form entrypoint with an exec-form wrapper
• allow commands such as bash or individual build scripts
• document the new container workflow

Closes #77
Closes #78

Testing

• git diff --cached --check
• docker build --check .
• docker build --target toolchain -t flipperone-toolchain:test .
• docker build --progress=plain -t flipperone-linux-build-scripts:test .
• verified that the final build reused the cached toolchain layers
• docker run --rm flipperone-linux-build-scripts:test bash -lc "echo override-ok; test -f ./build-uboot.sh"
• verified the command override printed override-ok and exited with status 0

[Yury-MonZon]

Small correction on the signal reasoning here: not exec-ing these doesn't orphan the build. Bash is waiting on each script as a foreground process, so a docker stop still tears the child down - it doesn't get abandoned.

Bash won't fire a trap until the current command returns and it won't reap zombies. If we want an instant, clean stop, simplest is docker run --init. Or maybe trap + background + wait.