fix: remove docker container after exit

[qwersem]

The build script leaves multiple containers in a host environment. This update adds an --rm option to remove a Docker container after a command is executed.

Steps to reproduce the issue

$ git clone https://github.com/riscv-software-src/riscv-unified-db.git
$ cd riscv-unified-db
$ export DOCKER=1
$ ./do test:smoke
Fetching gem metadata from https://rubygems.org/......
$ docker container ls -a
Emulate Docker CLI using podman. Create /etc/containers/nodocker to quiet msg.
CONTAINER ID  IMAGE                        COMMAND               CREATED         STATUS                     PORTS       NAMES
b49df1bcbc2c  docker.io/riscvintl/udb:0.9  bundle config set...  11 minutes ago  Exited (0) 11 minutes ago              quirky_swirles
28b906b730bf  docker.io/riscvintl/udb:0.9  bundle config set...  11 minutes ago  Exited (0) 11 minutes ago              amazing_goldberg
e189ebdd5e27  docker.io/riscvintl/udb:0.9  bundle config set...  11 minutes ago  Exited (0) 11 minutes ago              optimistic_kowalevski
9c34939c2770  docker.io/riscvintl/udb:0.9  bundle install        11 minutes ago  Exited (0) 10 minutes ago              magical_murdock
c118fdd21182  docker.io/riscvintl/udb:0.9  /usr/bin/python3 ...  10 minutes ago  Exited (0) 10 minutes ago              intelligent_margulis
bad9ac0863cf  docker.io/riscvintl/udb:0.9  /home/q/workspace...  10 minutes ago  Exited (0) 10 minutes ago              blissful_keller
c53716cb242a  docker.io/riscvintl/udb:0.9  npm i                 10 minutes ago  Exited (0) 10 minutes ago              cranky_colden
4eb3b575a6df  docker.io/riscvintl/udb:0.9  bundle exec --gem...  10 minutes ago  Exited (0) 5 minutes ago               nervous_germain