docker-images: make cadvisor build script executable
Created by: bobheadxi
Round 3. I messed it up again (https://buildkite.com/sourcegraph/sourcegraph/builds/64487#35588983-5395-47d2-a94f-a6ce3cd56ea5)
tl;dr try replicating steps locally
Follow-up to #10939 , #10938 ...