Pin Docker digests

This MR contains the following updates:

Package	Type	Update	Change
alpine	final	pin	3.12.0 -> 3.12.0
docker	service-image	pin	19.03.5-dind -> 19.03.5-dind
l.gcr.io/google/bazel	image-name	pin	3.3.1 -> 3.3.1

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

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