fix gradle caching in docker build (#4294)

## Description
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https://github.com/smithy-lang/smithy-rs/pull/4218 broke the docker
image build in a subtle way which is that we added Gradle build logic to
`buildSrc` that is used in the root `build.gradle.kts` file. This wasn't
found in PR:4218 because the docker build does a sparse checkout of the
`main` branch which did not have the new logic/issue and thus was not
triggered until after merging.

## Testing
<!--- Please describe in detail how you tested your changes -->
<!--- Include details of your testing environment, and the tests you ran
to -->
<!--- see how your change affects other areas of the code, etc. -->
Local + CI build succeeds

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