Modernize dockerfiles builds

[mik-laj]

I did the following changes:

• Bash code style unification (readonly variables, variable name convention)
• To build pgbouncer-exporter/Dockerfile, use multi-stage to simplify the building process and make it even more independent from the system
• Add a simple coherence check to improve confidence

CC: @pgvishnuram

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[pgvishnuram]

@mik-laj - would prefer to run the service as non root by default

[mik-laj]

Non-root will be nice, but in this PR I just wanted to refactor the code without changing the behavior.

[potiuk]

Nice!

[github-actions]

The PR is likely OK to be merged with just subset of tests for default Python and Database versions without running the full matrix of tests, because it does not modify the core of Airflow. If the committers decide that the full tests matrix is needed, they will add the label 'full tests needed'. Then you should rebase to the latest main or amend the last commit of the PR, and push it with --force-with-lease.