e2e Use useradd's --shell option

... in preference to `chsh`, since in recent alpine 3.9.2 images that can fail
with:

    Password: chsh: PAM: Authentication token manipulation error

Which seems to relate to the use of `!` as the password for `root` in `/etc/shadow`gq

Signed-off-by: Ian Campbell <ijc@docker.com>

e2e/testdata/Dockerfile.connhelper-ssh

@@ -1,10 +1,9 @@
 FROM docker:test-dind
 RUN apk --no-cache add shadow openssh-server && \
   groupadd -f docker && \
-  useradd -m penguin && \
+  useradd -m --shell /bin/sh penguin && \
   usermod -aG docker penguin && \
   usermod -p $(head -c32 /dev/urandom | base64) penguin && \
-  chsh -s /bin/sh penguin && \
   ssh-keygen -A
 # workaround: ssh session excludes /usr/local/bin from $PATH
 RUN  ln -s /usr/local/bin/docker /usr/bin/docker