When not using a ROOTCMD when running the test-setup-docker.sh the
script will fail with the following error message:
++ id -u
+ USER_ID=1000 docker-compose up -d
./test-setup-docker.sh: line 50: USER_ID=1000: command not found
Due to Bash's simple command expansion[0] variable assignments will be
interpreted as the command when no ROOTCMD is given. To work around this
we use default ROOTCMD to the `env` command.
[0]: https://www.gnu.org/savannah-checkouts/gnu/bash/manual/bash.html#Simple-Command-Expansion
Change-Id: Id696069a4eec2b2c2513ac449098b0ef73dc3906