Add mono-complete to Docker image.
Add mono-complete to Docker image.

After targeting .NET 4.7.1, some users started encountering errors like:

Could not find method '.ctor' due to a type load error: Could not load
file or assembly 'netstandard, Version=2.0.0.0, Culture=neutral,
PublicKeyToken=cc7b13ffcd2ddd51' or one of its dependencies.
assembly:/opt/duplicati/SharpAESCrypt.exe type:SharpAESCrypt
member:(null)

The TencentCOS and Tardigrade backends are built on .NET Standard 2.0,
and it appears that the dependencies can be awoken even if these
backends are not used.

This fixes #4289.

duplicati/duplicati,mnaiman/duplicati,mnaiman/duplicati,duplicati/duplicati,mnaiman/duplicati,mnaiman/duplicati,duplicati/duplicati,duplicati/duplicati,mnaiman/duplicati,duplicati/duplicati