build(deps): bump nokogiri from 1.14.1 to 1.15.6 in /tests/docker

Bumps [nokogiri](https://github.com/sparklemotion/nokogiri) from 1.14.1 to 1.15.6.
- [Release notes](https://github.com/sparklemotion/nokogiri/releases)
- [Changelog](https://github.com/sparklemotion/nokogiri/blob/main/CHANGELOG.md)
- [Commits](sparklemotion/nokogiri@v1.14.1...v1.15.6)

---
updated-dependencies:
- dependency-name: nokogiri
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

tests/docker/Gemfile.lock

@@ -10,12 +10,12 @@ GEM
     mime-types (3.4.1)
       mime-types-data (~> 3.2015)
     mime-types-data (3.2021.1115)
-    mini_portile2 (2.8.1)
+    mini_portile2 (2.8.5)
     netrc (0.11.0)
-    nokogiri (1.14.1)
-      mini_portile2 (~> 2.8.0)
+    nokogiri (1.15.6)
+      mini_portile2 (~> 2.8.2)
       racc (~> 1.4)
-    racc (1.6.2)
+    racc (1.7.3)
     rest-client (2.1.0)
       http-accept (>= 1.7.0, < 2.0)
       http-cookie (>= 1.0.2, < 2.0)