GH Actions - build-docker-images.yaml: allow testing image building without credentials

[romeroalx]

Short description

This PR allows not setting credentials when setting push: false. Useful for testing images.

Removes warning: LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 39)

Test: https://github.com/romeroalx/pdns/actions/runs/12885369941

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)
• checked that this code was merged to master

[coveralls]

Pull Request Test Coverage Report for Build 12890144520

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 45 unchanged lines in 10 files lost coverage.
• Overall coverage decreased (-0.001%) to 64.696%

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Files with Coverage Reduction	New Missed Lines	%
pdns/packethandler.cc	1	72.44%
modules/gpgsqlbackend/gpgsqlbackend.cc	1	88.62%
pdns/backends/gsql/gsqlbackend.hh	1	97.71%
pdns/recursordist/test-syncres_cc2.cc	3	88.85%
pdns/dnsdistdist/dnsdist-carbon.cc	3	64.27%
pdns/iputils.cc	3	56.07%
pdns/recursordist/recpacketcache.hh	3	89.55%
pdns/recursordist/test-syncres_cc1.cc	8	89.64%
pdns/recursordist/rec-tcp.cc	10	66.07%
ext/json11/json11.cpp	12	62.72%

Totals
Change from base Build 12884379895:	-0.001%
Covered Lines:	127797
Relevant Lines:	166433

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💛 - Coveralls