Export filenames that are supported in windows (and import those files properly as well)

Noticed while testing the docker export/import story that certain assets have names that include a colon, i.e. 'sha256:(someshavalue)`, and since we base the export filename off of the asset name, this fails on windows (where : isn't a supported filename character).

So we'll need some means to encode the filenames on export and not have these unsupported characters, as well as decoding them on import so proper asset names are used.