packaging: build: Make to be able to replace docker cli via environment variable

[cosmo0920]

Theoretically, podman cli could be used for creating packages. The default command for container cli should be docker.
To use podman instead of docker, define FLB_DOCKER_CLI=podman. Then podman is working with build and run commands.

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Testing
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• Example configuration file for the change
• Debug log output from testing the change

• Attached Valgrind output that shows no leaks or memory corruption was found

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

• Run local packaging test showing all targets (including any new ones) build.
• Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• Documentation required for this feature

Backporting

• Backport to latest stable release.

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

[cosmo0920]

Sorry, I'd overlooked for shellcheck failure. I re-requested for reviewing.