fix: set HOSTNAME variable to (re)enable server image indexing

Created by: rvantonder

Updates the HOSTNAME fix in the right place for docker images. See #6550 for details.

Test plan: Tested against the built docker image: https://buildkite.com/sourcegraph/sourcegraph/builds/48500#9a43fd21-5021-4585-b1d7-3eac0299fdbf