docs: use pre-built docker image for lsif-go

What does this MR do?

Recommends to use pre-built docker image instead of building the binary each time.

Reasons:

• Building master from scratch is not reproducible as repository is being developed. Docker image can be pinned to a tag or digest.
• go get is a way to add dependencies in Go. So if the project is written in Go dependency conflict might suddenly occur if there are shared dependencies.

Please see the discussion here gitlab-org/cluster-integration/gitlab-agent!68 (comment 401431566)

Related issues

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