Literally, this expression means "Fie, fie then!", but it's not as old-fashioned as "fie" has become in English. A typical French-to-English dictionary translation is "For shame!". Either "fi" or "fi donc" is used in French as a term of contempt, and the difference between them is subtle, perhaps something like "Crap!" vs. "What crap!" in English. Using them together just intensifies the expression by stretching it out, so "Fi, fi donc!" would correspond to "What a load of crap!" (However, "fi" is not crude like "merde", the more literal French equivalent of "crap".)
Lt. von Eyrick is a Prussian officer speaking French because he's part of an occupation force. His nickname "Mademoiselle Fifi" comes from his adoption of "Fi, fi donc!" as a pet expression, so it shouldn't be surprising if he uses it in contexts where it doesn't really apply. When setting off an improvised bomb for fun, something conveying the meaning "Quick, let's get out of here!" would be more appropriate than an expression of contempt.
