Pin Docker digests

Created by: renovate[bot]

This PR contains the following updates:

Package	Type	Update	New value	Sourcegraph
alpine	final	pin	3.11
golang	stage	pin	1.13-alpine

📌 Important: Renovate will wait until you have merged this Pin PR before creating any upgrade PRs for the affected packages. Add the preset :preserveSemverRanges your config if you instead don't wish to pin dependencies.

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