[AIRFLOW-3343] Update DockerOperator for Docker-py 3.0.0 API changes

[ashb]

Make sure you have checked all steps below.

Jira

• https://issues.apache.org/jira/browse/AIRFLOW-3343

Description

• When we updated the version of docker.py in [AIRFLOW-3203] Fix DockerOperator & some operator test #4049 we missed a breaking change. The API of wait() changed to return a dict, not just a number so this
  Operator wasn't actually working, but the tests were passing because the
  return was mocked in-correctly.

Since it appears that the python Docker module follows semver, with breaking changes only on major versions I have also changed the format specifier so we don't pull in 4.0.0 accidentally in the future.

I also removed shm_size from kwargs passed to BaseOperator to avoid
the deprecation warning about unknown args.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

Commits

• My commits all reference Jira issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • When adding new operators/hooks/sensors, the autoclass documentation generation needs to be added.

Code Quality

• Passes flake8

[ashb]

@XD-DENG PTAL - this was the bug I pinged you about in slack.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@a5f33f3). Click here to learn what that means.
The diff coverage is 100%.

@@           Coverage Diff            @@
##             master   #4187   +/-   ##
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  Coverage          ?   77.7%           
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  Files             ?     199           
  Lines             ?   16303           
  Branches          ?       0           
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  Hits              ?   12669           
  Misses            ?    3634           
  Partials          ?       0

Impacted Files	Coverage Δ
airflow/operators/docker_operator.py	97.67% <100%> (ø)

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[kaxil]

Minor Changes, LGTM otherwise.