refactor: Move Sentry source maps upload to Docker build step

Created by: plibither8

Should be merged after https://github.com/sourcegraph/sourcegraph/pull/39560

• refactor: Move Sentry source maps upload to Docker build step
• doc: remove sentry sourcemaps step from publish images

This PR refactors the current method and location for uploading source maps to Sentry.

Before:

• Source maps were uploaded in by a separate operation in the Buildkite pipline, under the "Publish images" operation set.
• Source maps were only uploaded on run-types that matched a Release branch build.

After:

• Source maps are uploaded while the Docker image is being built.
• Source maps are only uploaded if the run-type if of a Main branch build.

Test plan

Individually uploading a source map has been tested separately. Uploading on main branch build will be tested once merged.