[AIRFLOW-4086] Fixed pipefail - set bash shell in the current dockerfile

[potiuk]

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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-4086

Description

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  Fixed pipefail - set bash shell in the current dockerfile

Tests

• My does not need testing for this extremely good reason:
  Docker is not build in the current Travis. This might be changed when we discuss and merge
  [AIRFLOW-3718] [WIP/SPLIT] Multi-layered version of the docker image #4543
  Then Docker file will be evaluated and (incrementally) built on Travis

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Code Quality

• Passes flake8

[potiuk]

@ashb - I think this one should make it to 1.10.3. It's one liner fix that fixes Docker failing on DockerHub for the last 2 weeks or so.

[feng-tao]

lgtm

[feng-tao]

will merge the pr once CI finishes.