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 renzok/uid-map . 
docker run --name uid-map --rm -ti -e HOST_USER_ID=22222 -e HOST_USER_GID=22222 renzok/uid-map

or 

docker run --name uid-map --rm -ti -e HOST_USER_ID=$(id -u) -e HOST_USER_GID=$(id -g) -v ${HOME}/devel:/devel renzok/uid-map

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 "${HOST_USER_ID}" -a -z "${HOST_USER_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=${HOST_USER_ID:=$USER_ID}
USER_GID=${HOST_USER_GID:=$USER_GID}

LINE=$(grep -F "${USER}" /etc/passwd)
# replace all ':' with a space and create array
array=( ${LINE//:/ } )

# home is 5th element
USER_HOME=${array[4]}

sed -i -e "s/^${USER}:\([^:]*\):[0-9]*:[0-9]*/${USER}:\1:${USER_ID}:${USER_GID}/"  /etc/passwd
sed -i -e "s/^${USER}:\([^:]*\):[0-9]*/${USER}:\1:${USER_GID}/"  /etc/group

chown -R ${USER_ID}:${USER_GID} ${USER_HOME}

exec su - "${USER}"