Fix build by bumping nginx in server image to 1.18.0-r1
Created by: mrnugget
Build failure looks like 1.18.0-r0 has been bumped to 1.18.0-r1 in the alpine repository: https://buildkite.com/sourcegraph/sourcegraph/builds/76932#caad483b-e9b5-41dc-9077-cb091bd80769