TT#82852 Dockerfile: ensure to have procps present

Otherwise execution of FAI might fail:

| Calling task_faiend
| /usr/lib/fai/subroutines: line 142: ps: command not found
| [...]

This is supposed to be fixed with FAI 5.9.4, while
version 5.8.4 suffers from this bug, so until >=5.9.4
is available in buster/stable let's fix this via an
explicit dependency.

Change-Id: I99490f263d1b2a1aec65f55feebe429b62628918

grml_build/Dockerfile

@@ -5,7 +5,7 @@ FROM docker.mgm.sipwise.com/sipwise-buster:latest
 # is updated with the current date. It will force refresh of all
 # of the base images and things like `apt-get update` won't be using
 # old cached versions when the Dockerfile is built.
-ENV REFRESHED_AT 2020-06-04
+ENV REFRESHED_AT 2020-06-05
 
 # tools for building and testing
 RUN apt-get update && apt-get install --assume-yes --no-install-recommends \
@@ -25,6 +25,7 @@ RUN apt-get update && apt-get install --assume-yes --no-install-recommends \
   moreutils \
   mtools \
   pciutils \
+  procps \
   qemu-system-x86 \
   rsync \
   socat \