Rollup merge of #59253 - kennytm:precise-docker-cache-hash, r=pietroalbini

Calculate Docker cache hash precisely from Dockerfile's dependencies

#58549 changed the Docker cache calculation to include every file under `src/ci/docker`, so that when files under `dist-x86_64-linux` is changed, its dependent image `dist-i686-linux` will also be rebuilt.

However, this ultraconservative solution caused the `dist-i686-linux` to be rebuilt every time an irrelevant Dockerfile (e.g. the PowerPC ones) is changed, which increases the building time beyond 3 hours and forcing a spurious but expected failure.

This commit instead parses the Dockerfile itself and look for the actual dependencies. The scripts needs to be copied into the Docker image, which must be done with the COPY command, so we just need to find all lines with a COPY command and add the source file into the hash calculator.

Note: this script only handles single-lined COPY command in the form `COPY src1 src2 src3 dst`, since these are the only variant used inside this repository.

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