Use proper Docker images
Use proper Docker images

Previously we used

 * 'latest' docker image for xsnippet-web-backend which is not cool
   because will not produce reproducible application deployments.

 * xsnippetci/xsnippet-web-backend image which is obsolete as we decided
   to stick with more natural xsnippet/xsnippet-web-backend.

 * pretty old container of xsnippet-api which is at least 3 month old.

That's not the way we want to keep the things, so let's change it.

xsnippet/xsnippet-infra