[release 2.0.x] Fix docker steps on tagged builds.

[scottarnette]

Change Description

Logic that required a tagged build to function was finally exercised and exposed some errors when building eosio containers.

• Fixed incorrect tagging step to use BUILD_TAG instead of defaulting to latest.
• Add missing push step before removing the tag.

Change Type

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• Documentation

• Stability bug fix

• Other

• Other - special case

Testing Changes

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• New Tests

• Existing Tests

• Test Framework

• CI System

• Other

Consensus Changes

• Consensus Changes

API Changes

• API Changes

Documentation Additions

• Documentation Additions

[kj4ezj]

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