Avoid erasing the developers' Docker config on local builds

While working on #180 (closed), I wanted to build tasks locally, so I ran make build. It failed, because it lost the login credentials to gitlab.teklia.com: the build script overwrote my local ~∕.docker/config.json with nothing. I had to reset my personal access tokens and log in again.

I think the CI is already using the built-in GitLab dependency proxy already; isn't this part of the script unnecessary?