Hi,

your docker image seems to be missing the 'system-user-nobody' package. This is a dependency for 'stunnel', without it you won't be able to install without breaking that dependency (Solution 2: break stunnel-5.49-37.14.x86_64 by ignoring some of its dependencies).
I was able to reproduce your issue on a sles15-sp1 virtual machine by removing the package and trying to reinstall stunnel:

Code:
test2:~ # zypper rm system-user-nobody
[...]
Resolving package dependencies...

The following package is going to be REMOVED:
  system-user-nobody

1 package to remove.
After the operation, 116.0 B will be freed.
Continue? [y/n/v/...? shows all options] (y): 
(1/1) Removing system-user-nobody-20170617-4.155.noarch .................................................................................................................................................................................................................[done]

test2:~ # zypper ar http://download.opensuse.org/repositories/security:/Stunnel/SLE-15/ stunnel
test2:~ # zypper ref

test2:~ # zypper in stunnel
Loading repository data...
Reading installed packages...
Resolving package dependencies...

Problem: nothing provides group(nogroup) needed by stunnel-5.49-35.1.x86_64
 Solution 1: do not install stunnel-5.49-35.1.x86_64
 Solution 2: break stunnel-5.49-35.1.x86_64 by ignoring some of its dependencies

Choose from above solutions by number or cancel [1/2/c] (c): 

test2:~ # zypper mr -e 1    
Repository 'SLES15-SP1-15.1-0' has been successfully enabled.

test2:~ # zypper in stunnel
Loading repository data...
Reading installed packages...
Resolving package dependencies...

The following 2 NEW packages are going to be installed:
  stunnel system-user-nobody

The following package has no support information from its vendor:
  stunnel

2 new packages to install.
Overall download size: 178.7 KiB. Already cached: 0 B. After the operation, additional 292.0 KiB will be used.
Continue? [y/n/v/...? shows all options] (y): 
Retrieving package system-user-nobody-20170617-4.155.noarch                                                                                                                                                                               (1/2),  10.4 KiB (  116   B unpacked)
Retrieving package stunnel-5.49-35.1.x86_64                                                                                                                                                                                               (2/2), 168.3 KiB (291.9 KiB unpacked)
Retrieving: stunnel-5.49-35.1.x86_64.rpm ...................................................................................................................................................................................................................[done (12.6 KiB/s)]
Checking for file conflicts: ............................................................................................................................................................................................................................................[done]
(1/2) Installing: system-user-nobody-20170617-4.155.noarch ..............................................................................................................................................................................................................[done]
Additional rpm output:
groupadd -r -g 65533 nogroup                                                                                                                                                                                                                                                   
groupadd -r -g 65534 nobody                                                                                                                                                                                                                                                    
useradd -r -s /sbin/nologin -c "nobody" -g nobody -d /var/lib/nobody -u 65534 nobody                                                                                                                                                                                           
usermod -a -G nogroup nobody                                                                                                                                                                                                                                                   
usermod: no changes                                                                                                                                                                                                                                                            


(2/2) Installing: stunnel-5.49-35.1.x86_64 ..............................................................................................................................................................................................................................[done]
So in conclusion your docker image seems to be corrupted or falsely installed, the package 'system-user-nobody' is part of the minimal-base-system. After re-enabling the sles iso as a repo the dependency can be resolved and 'system-user-nobody' is reinstalled.

Regards,
Eugen