Update scripts to work with podman

[kim-tsao]

Please specify the area for this PR
/area registry

What does does this PR do / why we need it:
Updates the scripts and READMEs to use podman as an alternative. In order to have the alias command to run consistently, I had to source the scripts in ./build_registry.sh so they would all run under the same shell.

Which issue(s) this PR fixes:

Fixes #?
devfile/api#766

PR acceptance criteria:

• Test (WIP)

• Documentation (WIP)

How to test changes / Special notes to the reviewer:

set the environment variable USE_PODMAN=true and run the main build script ./build_registry.sh

[openshift-ci]

@kim-tsao: The label(s) area/registry cannot be applied, because the repository doesn't have them.

In response to this:

Please specify the area for this PR
/area registry

What does does this PR do / why we need it:
Updates the scripts and READMEs to use podman as an alternative. In order to have the alias command to run consistently, I had to source the scripts in ./build_registry.sh so they would all run under the same shell.

Which issue(s) this PR fixes:

Fixes #?
devfile/api#766

PR acceptance criteria:

• Test (WIP)

• Documentation (WIP)

How to test changes / Special notes to the reviewer:

set the environment variable USE_PODMAN=true and run the main build script ./build_registry.sh

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[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: johnmcollier, kim-tsao

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