ci: fix docker cache hash collision
#58416 uncovered a bug in our caching for docker images: if the image `foo` pulls files from the image `bar` and a file in `bar` changed, the hash of `foo` will be the same even though it should be different. In that PR's case, `dist-i686-linux` pulls scripts from `dist-x86_64-linux`, and the PR only changed those scripts, causing an hash collision for `dist-i686-linux`.
We have to fix this, since the image will be rebuilt every time bors switches from testing master to testing beta/stable (and when it switches back), making CI way more painful than it currently is.
The approach used by this PR is to just include all the files in `src/ci/docker` in the hash. It's a bit heavy-handed and it will cause a rebuild of all the images every time a single image changes, but it's the best I can think of.
r? @Mark-Simulacrum
cc @alexcrichton @kennytm
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