Resolve Entrypoint and Cmd if there is no command and no args

[vdemeester]

Changes

Prior to this change, if a Step specify neither command nor args,
the entrypoint resolver would only resolve the image Entrypoint and
thus, only executing that entrypoint. This is confusing to user as it
is not how a docker run without args produces.

This make the behavior very close to docker run. If there is no
command and no args specified, the entrypoint reslover will join the
image Entrypoint and Cmd.

Signed-off-by: Vincent Demeester vdemeest@redhat.com

/kind bug

Fixes #4794

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• Docs included if any changes are user facing
• Tests included if any functionality added or changed
• Follows the commit message standard
• Meets the Tekton contributor standards (including
  functionality, content, code)
• Release notes block below has been filled in
  (if there are no user facing changes, use release note "NONE")

Release Notes

The entrypoint resolve will now reslove `Entrypoint` and `Cmd` in case
the steps has no command and no args specified.

[vdemeester]

/cc @abayer @imjasonh

[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-pipeline-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/pod/entrypoint_lookup.go	85.7%	86.4%	0.6
pkg/pod/entrypoint_lookup_impl.go	74.2%	75.0%	0.8

[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-pipeline-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/pod/entrypoint_lookup.go	85.7%	86.4%	0.6
pkg/pod/entrypoint_lookup_impl.go	74.2%	75.0%	0.8

[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-pipeline-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/pod/entrypoint_lookup.go	85.7%	86.4%	0.6
pkg/pod/entrypoint_lookup_impl.go	74.2%	75.0%	0.8

[abayer]

/lgtm

[abayer]

/lgtm

[abayer]

/retest

[imjasonh]

/test pull-tekton-pipeline-go-coverage

[imjasonh]

/approve

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: imjasonh

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[abayer]

/retest

[tekton-robot]

@vdemeester: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
pull-tekton-pipeline-go-coverage	f7a6c03	link	/test pull-tekton-pipeline-go-coverage

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