We would like this notice to be displayed on wikitech-static:
You are browsing a read-only backup copy of Wikitech. The primary site can be found at [https://wikitech.wikimedia.org wikitech.wikimedia.org]
When we have an active sitenotice for the main wikitech site this can be done with some parser function magic:
<!-- NOTE: replace page content with "-" when removing notice from Wikitech --> {{#ifeq: {{SERVERNAME}} | wikitech-static.wikimedia.org | <!--special banner for wikitech-static--> You are browsing a read-only backup copy of Wikitech. The primary site can be found at [https://wikitech.wikimedia.org wikitech.wikimedia.org] | <!--normal banner for actual wikitech--> ... }}
There is a bugfeature however when we don't have site notice text on the main site that makes MediaWiki display an empty div and the [dismiss] link. This is gross and makes @Krinkle sad, so lets find another way.
@Legoktm pointed out that we can set $wgSitenotice in LocalSettings. This will be displayed unless MediaWiki:Sitenotice is also non-empty. We will need to keep this in mind when posting a MediaWiki:Sitenotice banner and then remember to add the parser function magic back in.