Add dumb-init as the entrypoint for the default flytekit docker image

[EngHabu]

Tracking issue

https://github.com/flyteorg/flyte/issues/

Why are the changes needed?

What changes were proposed in this pull request?

How was this patch tested?

Setup process

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Check all the applicable boxes

• [ ] I updated the documentation accordingly.
• [ ] All new and existing tests passed.
• [ ] All commits are signed-off.

Related PRs

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (e107994) 85.71% compared to head (06bda9f) 85.71%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2073   +/-   ##
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  Coverage   85.71%   85.71%           
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  Files         313      313           
  Lines       23396    23396           
  Branches     3501     3501           
=======================================
  Hits        20055    20055           
  Misses       2733     2733           
  Partials      608      608           

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[kumare3]

is this for correctly propagating signals?