chore: Added cached layer for Go dependencies to Dockerfiles

[hbelmiro]

Description of your changes:
This PR modifies the Dockerfiles to download dependencies earlier. That way the download layers can be cached and subsequent builds will be faster when dependencies don't change.

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[google-oss-prow]

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[VaniHaripriya]

/lgtm

[DharmitD]

/lgtm

[HumairAK]

/approve

[google-oss-prow]

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This pull-request has been approved by: HumairAK

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