Run a shell inside a shell-less docker image (Ubuntu host, ephemeral container)

docker-shell.sh

#!/bin/bash

# ++++++++++++++++++++++

die() {
    echo -e "\033[31;1m$*\033[0m"
    exit 1
}

# ++++++++++++++++++++++


do_run() {
    docker run -it -v "$HBBOX:/$GBBOXd/busybox" --entrypoint="/$GBBOXd/busybox" "$@" "$TARGET" sh
}

do_exec() {
    docker cp "$HBBOX" "$TARGET:/$GBBOXd/busybox"
    docker exec -it "$@" "$TARGET" "/$GBBOXd/busybox" sh
}

# ++++++++++++++++++++++

printhelp() {
cat <<EOHELP

Adds busybox to an image or a running container, and runs an interactive shell inside the container.

    $0 { run IMAGE | exec CONTAINER } BOXDIR [DOCKER_ARGS ...]

run IMAGE : run the shell in a new container from IMAGE

exec CONTAINER : run the shell in a running CONTAINER

BOXDIR : where to copy busybox to in the container - e.g. "/bin"
        You may need to set this to a user directory if container is not running as root


Example:

    $0 run hello-world /bin --rm

Runs /bin/busybox in an ephemeral container ('--rm' is passed to 'docker run', not to the image entrypoint)

EOHELP
}

# ++++++++++++++++++++++

main() {
    if [[ "$*" =~ --help ]]; then
        printhelp
        exit 0
    fi

    ACTION="${1:-}"; shift || die "No action specified (run/exec)"

    TARGET="${1:-}"; shift || die "No image/container specified"

    HBBOX="$(which busybox)" || die "Busybox not in \$PATH on host."
    
    GBBOXd="${1:-}" ; shift || die "You must indicate a full path to a directory in the container into which to copy busybox\n\n\te.g. '/home/user'"

    case "$ACTION" in
        exec)
            do_exec "$@"
            ;;
        run)
            do_run "$@"
            ;;
        *)
            die "Not a supported action '$ACTION'"
            ;;
    esac
}

main "$@"

At this point I should announce this script is released GPLv3.

Works so long as bash and busybox are both available on the host system. installing busybox does not replace the utilites it would normally substitute for if they are already present.