It won't be that simple.<...> a simpple way to rip out the movie out of eht mkv container <...>
You see, the Linux community would happily use the AVI format, if it weren't so limiting to "modern" features such as variable-bitrate audio or sub-titles. If you have a mkv container, you bet 99% that it was packaged that way because it needs something than an AVI container could not hold.
If it really needs to be, VirtualDUB is recommended for the conversion because it has the "Direct stream copy" option to at least pass the video stream through unharmed.