fix(docker build): update index checksum

[sambacha]

Fixes #7925

This is needed to ensure that we have computed the trailing checksum for the index.
index.skipHash=true speeds up index writes by not computing a trailing checksum¹.
This is a result of having features.manyFiles enabled and cargo computing this for fingerprint².

Confirmed on macos-arm64 ea2eff95b@master

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Footnotes

1. see https://git-scm.com/docs/git-config#Documentation/git-config.txt-featuremanyFiles ↩

2. see https://doc.rust-lang.org/nightly/nightly-rustc/cargo/core/compiler/fingerprint/index.html ↩

[DaniPopes]

lgtm