cli: tweak installer to allow spaces in step IDs (#897)

For the script that gets generated by the installer, the call to `docker`
must receive the command-line arguments verbatim, including
quotes. Therefore `$@` needs to be quoted.

Without quotes (before):
$ popper run -f wf.yml "first step"
would execute:
$ docker run <...> getpopper/popper:v2.7.0 run -f wf.yml first step

With quotes (after fix):
$ docker run <...>  getpopper/popper:v2.7.0 run -f wf.yml "first step"

[skip ci]

install.sh

@@ -45,7 +45,7 @@ docker run --rm -ti \
   --volume "$PWD":"$PWD" \
   --workdir "$PWD" \
   --env-file /tmp/.envfile \
-  getpopper/popper:v2.7.0 $@
+  getpopper/popper:v2.7.0 "$@"
 EOF
 
 if [ "$?" -eq 0 ]; then