Phpmyadmin 4.x image availability

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This PR fixes a bug where phpmyadmin 4.x versions would fail to pull because their Docker tags are only available on the deprecated phpmyadmin/phpmyadmin image, not the official phpmyadmin image. The builder now conditionally uses the correct image based on the specified version, and the documentation has been updated to reflect this.

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Automatically closed this PR because it was created by Bugbot autofix for #74, and that PR was closed.