complete --shell-cmd support parts to get it working

osc/build.py

@@ -725,6 +725,10 @@ def main(apiurl, opts, argv):
     if opts.shell:
         buildargs.append("--shell")
 
+    if opts.shell_cmd:
+        buildargs.append("--shell-cmd")
+        buildargs.append(opts.shell_cmd)
+
     if opts.noinit:
         buildargs.append('--noinit')
 

osc/commandline.py

@@ -6406,10 +6406,14 @@ Please submit there instead, or use --nodevelproject to force direct submission.
 
             osc shell [OPTS] REPOSITORY ARCH
 
+        Run a single command inside of the build environment:
+            osc shell --shell-cmd=COMMAND [OPTS] REPOSITORY ARCH
+
         OPTS may be
 
             --noinit             # for faster run
             --shell-cmd=COMMAND
+            --extra-pkgs=PACKAGE # install additional packages
 
         To clean up the build environment run