[DOCS] Using ImageSpec in ContainerTask

[machichima]

Tracking issue

Couple with this PR in flytesnacks, which provides the example code of using ImageSpec in the ContainerTask.

Why are the changes needed?

Using ImageSpec in ContainerTask is not documented, and is important if we want to run the files that are not in the image.

What changes were proposed in this pull request?

How was this patch tested?

Setup process

Screenshots

Check all the applicable boxes

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

Related PRs

Docs link

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 37.10%. Comparing base (9028375) to head (c43ea77).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6095   +/-   ##
=======================================
  Coverage   37.10%   37.10%           
=======================================
  Files        1318     1318           
  Lines      132403   132403           
=======================================
+ Hits        49122    49126    +4     
+ Misses      79009    79005    -4     
  Partials     4272     4272           

Flag	Coverage Δ
unittests-datacatalog	51.58% <ø> (ø)
unittests-flyteadmin	54.10% <ø> (ø)
unittests-flytecopilot	30.99% <ø> (ø)
unittests-flytectl	62.33% <ø> (+0.04%)	⬆️
unittests-flyteidl	7.23% <ø> (ø)
unittests-flyteplugins	53.82% <ø> (ø)
unittests-flytepropeller	42.60% <ø> (ø)
unittests-flytestdlib	57.59% <ø> (ø)

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