use /bin/sh instead of /bin/bash in order to support dockerfile images that don't include bash. fixes #1364

plugins/common/functions

@@ -134,7 +134,7 @@ verify_image() {
 is_image_buildstep_based() {
   # circleci can't support --rm as they run lxc in lxc
   [[ ! -f "/home/ubuntu/.circlerc" ]] && local DOCKER_ARGS="--rm"
-  docker run --entrypoint="/bin/bash" $DOCKER_ARGS "$@" -c "[[ -f /exec ]]"
+  docker run --entrypoint="/bin/sh" $DOCKER_ARGS "$@" -c "test -f /exec"
 }
 
 is_number() {