automation/build: update stretch-i386 dockerfile

automation/build: update stretch-i386 dockerfile

We don't need to specify /bin/bash in the entry point rune, otherwise
non-interactive invocation of the container would fail with something
like:

+ C=debian:stretch-i386
+ export CONTAINER=registry.gitlab.com/xen-project/xen/debian:stretch-i386
+ CONTAINER=registry.gitlab.com/xen-project/xen/debian:stretch-i386
+ cd /local/work/COMMITTER/xen-32.git
+ git fetch origin
+ con git reset --hard origin/staging
*** Ensuring registry.gitlab.com/xen-project/xen/debian:stretch-i386 is up to date
*** Launching container ...
/usr/bin/git: /usr/bin/git: cannot execute binary file

While at it, use shorthand "linux32".

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Doug Goldstein <cardoe@cardoe.com>