Feature/ubuntu doc adjustment

[delight]

Adjusts install.md (Ubuntu):

• registries.conf setup is now in containers-image package
• replaces ${NAME} with hard-encoded Ubuntu to support all *buntu flavors
• combines downloading and registering of the apt-secure key into a piped oneliner

[openshift-ci-robot]

Hi @delight. Thanks for your PR.

I'm waiting for a containers member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[vrothberg]

@lsm5 PTAL

[rhatdan]

LGTM

[baude]

/approve

[baude]

/lgtm

[mheon]

/ok-to-test
/approve
LGTM

[lsm5]

LGTM, the apt-key-add simplification can be used for other debian / raspbian sections too for consistency, but either way works..

[openshift-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: baude, delight, lsm5, mheon

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