[BEAM-7909] fix python container image name

[Hannah-Jiang]

Fix Python container image name variable.

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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	---		---	---

See .test-infra/jenkins/README for trigger phrase, status and link of all Jenkins jobs.

[Hannah-Jiang]

Run Python Dataflow ValidatesContainer

[Hannah-Jiang]

Run Python PreCommit

[Hannah-Jiang]

R: @aaltay
I found a typo when running post commit tests. Is it possible to review and merge?

Thanks,