docker: mapping host uid and gid to user inisde container

Dockerfile

FROM debian:jessie

ENV USER=boatswain USER_ID=1000 USER_GID=1000

# now creating user
RUN groupadd --gid "${USER_GID}" "${USER}" && \
    useradd \
      --uid ${USER_ID} \
      --gid ${USER_GID} \
      --create-home \
      --shell /bin/bash \
      ${USER}

COPY user-mapping.sh /
RUN  chmod u+x user-mapping.sh

ENTRYPOINT ["/user-mapping.sh"]

example use

docker build -t imageName . 
docker run --name uid-map --rm -ti -e UID=22222 -e GID=22222 imageName

user-mapping.sh

#!/bin/bash

if [ -z "${USER}" ]; then
  echo "We need USER to be set!"; exit 100
fi

# if both not set we do not need to do anything
if [ -z "${UID}" -a -z "${GID}" ]; then
    echo "Nothing to do here." ; exit 0
fi

# reset user_?id to either new id or if empty old (still one of above
# might not be set)
USER_ID=${UID:=$USER_ID}
USER_GID=${GID:=$USER_GID}

usermod  -u USER_ID  ${USER}    
groupmod -g USER_GID ${USER}
usermod  -g USER_ID  ${USER}

exec su - "${USER}"