Run docker-compose up test setup script with env

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: I3e4dd844efae8971d9ba83060a7134f1f1f0e770
Co-Authored-By: Simon Westphahl <simon.westphahl@bmw.de>

tools/test-setup-docker.sh

@@ -8,7 +8,11 @@
 # environment.
 
 set -xeu
-ROOTCMD=${ROOTCMD:-}
+
+# Default ROOTCMD to the 'env' command, otherwise variable assignments will be
+# interpreted as command when no ROOTCMD is given. The reason for that is
+# Bash's simple command expansion.
+ROOTCMD=${ROOTCMD:-env}
 
 cd $(dirname $0)
 SCRIPT_DIR="$(pwd)"