fix: [base-0k] Setting a default empty value for DOCKER

Avoid ``set -eux`` complaining about variable not set when launched
from various script. (Docker setup failed and lxc-script failed and
even direct usage will fail without this.)

precise/base-0k/hooks/install

@@ -2,6 +2,8 @@
 
 set -eux # -x for verbose logging to juju debug-log
 
+DOCKER=${DOCKER:-}
+
 ## 0k git remote path
 GIT_0K_BASE=${GIT_0K_BASE:-"git.0k.io:/var/git"}