Replace docker-fixtures with testcontainers

[strangelookingnerd]

Replace docker-fixtures with testcontainers (see jenkinsci/docker-fixtures#122 (comment))

Testing done

mvn clean verfiy had failing tests with No such command: dist-fork on my machine.

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed

[basil]

This PR is introduced technical debt, as the Dockerfile is duplicated between jenkinsci/workflow-api-plugin#414 and jenkinsci/docker-fixtures#122, which creates a maintenance burden. See this page for more information about why this is the case. I recommend refactoring the common images that used to be present in docker-fixtures into a new Jenkins repository for TestContainers, something like https://github.com/dasniko/testcontainers-keycloak.

[dduportal]

(Note: Windows agents are not allocated due to deprecated label. Please see #16 for a fix)