Update docker Docker tag to v20.10.21

This MR contains the following updates:

Package	Type	Update	Change
docker	image	patch	20.10.17 -> 20.10.21

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.gitlab-ci.yml

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-image: docker:20.10.17
+image: docker:20.10.21
 
 variables:
   DOCKER_TLS_CERTDIR: "/certs"