[Autoscaler] Limit additional fields in available_node_types::docker

[ijrsvt]

Why are these changes needed?

• We should not allow arbitrary fields to be set here (the default is additionalProperties = True).

Related issue number

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[AmeerHajAli]

Not sure why TUNE_TESTING is failing. CC @richardliaw