Pin ODM image to 3.5.6 until 3.6.0 build fixed

[smathermather]

Per https://community.opendronemap.org/t/processing-stopped-because-of-strange-values-in-the-reconstruction-gdal-translate-failing-on-dem-hillshade/25880

When I run ODM without --dsm and --dtm, everything completes successfully and I get a good orthophoto. As soon as I turn those two flags on, the job runs fine until 81% and then crashes with this error:

SubprocessException: Child returned 1
[ERROR] Uh oh! Processing stopped because of strange values in the reconstruction.