Support singularity run

[rujinlong]

In the docs/container.rst file, it said

In practice Nextflow will automatically wrap your processes and launch them by running the singularity exec command with the image you have provided.

My question is, is it possible to set nextflow to run singularity run, not singularity exec? The benefit of using run is when the container is built based on micromamba-docker, we can have multiple conda/mamba env in a container. By setting containerOptions = "--env ENV_NAME=env1", the process will run in the env1 environment.

For docker this is not a problem, since nextflow call docker run by default. It will be really helpful if singularity run is also supported.

[stale]

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