[AIRFLOW-1382] Add working dir option

[benjaminsims]

Allow the user to specify the working directory to be used in the
created container. Equivalent to docker run/create -w.

Dear Airflow maintainers,

Please accept this PR. I understand that it will not be reviewed until I have checked off all the steps below!

JIRA

• My PR addresses the following Airflow JIRA issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-1382

Description

• Here are some details about my PR, including screenshots of any UI changes:

Docker containers can have a default working directory set, done using docker run/create -w on the command line. This PR updates the DockerOperator to provide that option.

Tests

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

Parameter added into existing unit test.

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
  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"

[mention-bot]

@benjaminsims, thanks for your PR! By analyzing the history of the files in this pull request, we identified @asnir, @vitorbaptista and @artwr to be potential reviewers.

[codecov-io]

Codecov Report

Merging #2419 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2419      +/-   ##
==========================================
- Coverage   69.25%   69.24%   -0.01%     
==========================================
  Files         146      146              
  Lines       11230    11231       +1     
==========================================
  Hits         7777     7777              
- Misses       3453     3454       +1

Impacted Files	Coverage Δ
airflow/operators/docker_operator.py	97.4% <100%> (+0.03%)	⬆️
airflow/models.py	87.26% <0%> (-0.05%)	⬇️

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