Add requirements for container images to run with Nextflow (#3586)

Signed-off-by: Paolo Di Tommaso <paolo.ditommaso@gmail.com>
Signed-off-by: Marcel Ribeiro-Dantas <mribeirodantas@seqera.io>

docs/container.rst

@@ -11,6 +11,10 @@ platform that supports a container runtime. Furthermore, the same pipeline can b
 executed with any of the supported container runtimes, depending on which runtimes are available
 in the target compute environment.
 
+.. note::
+  When creating your container image to use with Nextflow make sure it includes Bash (3.x or later) and ``ps`` are installed in your
+  image, along with other tools required for collecting metrics (See the list :ref:`here <trace-required-packages>`). Also, Bash should be available on the path `/bin/bash` and it should be the container entry-point. 
+
 .. _container-charliecloud:
 
 Charliecloud

docs/tracing.rst

@@ -136,6 +136,8 @@ and many other metrics. You can see an example below:
 
 .. image:: images/report-tasks-min.png
 
+.. _trace-required-packages:
+
 .. note::
   Nextflow collects these metrics through a background process for each job in the target environment.
   Make sure the following tools are available in the environment where tasks are executed: ``awk``, ``date``, ``grep``, ``ps``, ``sed``, ``tail``, ``tee``.