The flaky test is `"allowlists a site based on a $domain"`. It adds two filters
at the same time, one that blocks a resource and one `$document` filter for the
page the resource is on. In MV3, these are added simultaneously in a single call
to `declarativeNetRequest.updateDynamicRules`. Unfortunately, as we've seen in
the allowlisting tests, Edge in MV3 sometimes takes a moment for the
document-level allowlisting filters to take effect. This seems to be true even
if it is added simultaneously with other filters.

This MR adds a retry on the assertion for this test, so that Edge can be
eventually consist with its DNR rules.

The MR also touches some minor details I ran into while fixing and testing this:
- Some docs in the cheat sheet still referred to `mv3` instead of just `3` for
  the functional test script.
- The caching of the npm install step in docker images has been updated to match
  the pattern in the monorepo.
- The .dockerignore file has been updated to also exclude node modules / browser
  snapshots in the core directory.