A single bash script that bootstraps a container, then runs itself inside that container. Use any "FROM" line. The only requirement is that /bin/bash is present. See also [Turdokken](https://github.com/zsteinkamp/turdokken).

dockerscript

#!/bin/bash
  
# Shell code to run in the container
cmd () {
  echo "Hello, ${OUTER_USER}. I am $(whoami)."
  cat /etc/issue
}

# Dockerfile definition
read -r -d '' DOCKERFILE <<EOD
FROM alpine:3.12
RUN apk add --no-cache bash
###### lines below are required ######
ENV IN_CONTAINER=true
ENTRYPOINT ["/runme"]
EOD

# error out
set -e

# Logic to build/run the container or run the script
if [[ -z $IN_CONTAINER ]]; then
  COMMAND_FILE="$0"
  if [[ "$COMMAND_FILE" != /* ]]; then
    # Add the current dir to COMMAND_FILE if we only have the relative path
    COMMAND_FILE="$PWD/$COMMAND_FILE"
  fi
  COMMAND_FILE_BASE=$(basename "$COMMAND_FILE")
  TAG="dockerscript-$(basename ${COMMAND_FILE_BASE//[^A-Za-z0-9-_]/})";
  echo "Building container..." 1>&2
  docker build --quiet --tag $TAG - <<< "${DOCKERFILE}" 1>&2
  echo "Running container..." 1>&2
  echo 1>&2
  exec docker run --rm -it -e OUTER_USER=${USER} --mount type=bind,source="${COMMAND_FILE}",target=/runme $TAG $@
else
  cmd ## run the script
fi