Ubuntu-based Docker image uses outdated pysam, incompatible with Tools/bamstats.py

[bernochoa]

The default ubuntu-based docker image is built using the default Python libraries from Ubuntu's repo, which for the pysam library are very outdated. This breaks the Tools/bamstats.py module, which calls pysam.idxstats(). This function previously returned a list instead of the string that the module expects, leading to an attribute exception.

[pkrusche]

Thanks! I'll change the pysam method of installation to pip for the next release.