Make it possible to user Docker images with --privileged flag

I'm currently trying to use Pipelines to build a Singularity container (https://singularity.lbl.gov/docs-build-container) which will later either be pushed or a remote server or available as a download. I have a Docker image I'm using a build environment, however the when the step comes to build the Singularity container an error is thrown:

+ singularity build shiny.img shiny.def
Using container recipe deffile: shiny.def
Sanitizing environment
ERROR : Could not virtualize mount namespace: Operation not permitted
ABORT : Retval = 255
Cleaning up...

Apparently the way around this to run Docker with the --privileged flag (see https://github.com/singularityware/singularity/issues/632#issuecomment-296545074). After having spoken with support it's come to my attention this is currently impossible with Pipelines (see https://confluence.atlassian.com/bitbucket/run-docker-commands-in-bitbucket-pipelines-879254331.html). It would be useful to have this option available for builds in which it is required.

• https://singularity.lbl.gov/docs-build-container
• https://github.com/singularityware/singularity/issues/632