fix(config): Restore old environment var behavior with option

[srebhan]

• Updated associated README.md.
• Wrote appropriate unit tests.
• Pull request title or commits are in conventional commit format

resolves #13456
resolves #13432
resolves #13455
resolves #13460

After a lot of back-and-forth this PR restores the old environment-variable behavior using the --old-env-behavior command line option. That is, all $VAR and ${VAR} environment variables are replaced if and only if they do exist, otherwise those patterns are left untouched. Furthermore, double dollar signs $$ do not have any meaning so $$VAR becomes $foobar if VAR="foobar" in the environment.

If the new behavior is enabled (default), the only difference is that $$ will escape the expression, i.e. $$VAR will become $VAR even if VAR is defined in the environment...

Here an exemplary overview of the behavior with VAR="foo" and FALLBACK="bar" defined in the environemt:

input	result with --old-env-behavior	result without --old-env-behavior	identical
$VAR	foo	foo	✔️
$$VAR	$foo	$VAR	✖️
\$VAR	\foo	\foo	✔️
${VAR}	foo	foo	✔️
$${VAR}	$foo	${VAR}	✖️
\${VAR}	\foo	\foo	✔️
\$${VAR}	\$foo	\${VAR}	✖️
$FOO	$FOO	$FOO	✔️
${FOO}	${FOO}	${FOO}	✔️
$${FOO}	$${FOO}	${FOO}	✖️
${FOO:-${FALLBACK}}	bar	bar	✔️
${FOO:-FALLBACK}	FALLBACK	FALLBACK	✔️

[srebhan]

Please note that the replacement in go.mod can be removed as-soon-as compose-spec/compose-go#418 is merged.

[telegraf-tiger]

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