[BEAM-8165] Revert docker default root name

[Hannah-Jiang]

Use apachebeam only for sdk harness images.

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Post-Commit Tests Status (on master branch)

Lang	SDK	Apex	Dataflow	Flink	Gearpump	Samza	Spark
Go		---	---		---	---
Java
Python		---			---	---
XLang	---	---	---		---	---	---

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website
Non-portable
Portable	---		---	---

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[Hannah-Jiang]

R: @aaltay
Cc: @markflyhigh, we need to cherry pick this PR.

[aaltay]

Could you add more information to here or the JIRA about:

• What is happening here? When is docker-repository-root defined and when it is not?
• What is it fixing?
• Why do we use a mix of bintray and docker hub?

[Hannah-Jiang]

Could you add more information to here or the JIRA about:

• What is happening here? When is docker-repository-root defined and when it is not?
• What is it fixing?
• Why do we use a mix of bintray and docker hub?

I added a comment to the ticket.

[aaltay]

LGTM. Thank you. After cherry pick let's clean up this 2 root thing.