If you run "dbuild" on a freshly installed machine, the error message is
not the most helpful one. Fix it up.
Before:
$ ./tools/toolchain/dbuild
./tools/toolchain/dbuild: line 113: docker: command not found
./tools/toolchain/dbuild: line 156: docker: command not found
After:
$ ./tools/toolchain/dbuild
dbuild: Please install Docker on this machine to run dbuild.
Run `./tools/toolchain/dbuild --help' to print the full help message.
Message-Id: <20200426192746.11034-1-penberg@scylladb.com>
fad6712673