Added functionality to ease the use of X image

Functionality borrowed from the Raspberry Pi cross compile docker build
file from https://github.com/sdt/docker-raspberry-pi-cross-compiler

Dockerfile

@@ -51,3 +51,8 @@ RUN git clone https://github.com/martine/ninja.git && \
   ./ninja && \
   cp ./ninja /usr/bin/ && \
   cd .. && rm -rf ninja
+
+WORKDIR /build
+ENTRYPOINT ["/dockcross/entrypoint.sh"]
+
+COPY imagefiles/entrypoint.sh imagefiles/dockcross /dockcross/

imagefiles/dockcross

@@ -0,0 +1,188 @@
+#!/bin/bash
+
+#------------------------------------------------------------------------------
+# Helpers
+#
+err() {
+    echo -e >&2 ERROR: $@\\n
+}
+
+die() {
+    err $@
+    exit 1
+}
+
+has() {
+    # eg. has command update
+    local kind=$1
+    local name=$2
+
+    type -t $kind:$name | grep -q function
+}
+
+#------------------------------------------------------------------------------
+# Command handlers
+#
+command:update-image() {
+    docker pull $FINAL_IMAGE
+}
+
+help:update-image() {
+    echo Pull the latest $FINAL_IMAGE .
+}
+
+command:update-script() {
+    if cmp -s <( docker run $FINAL_IMAGE ) $0; then
+        echo $0 is up to date
+    else
+        echo -n Updating $0 '... '
+        docker run $FINAL_IMAGE > $0 && echo ok
+    fi
+}
+
+help:update-image() {
+    echo Update $0 from $FINAL_IMAGE .
+}
+
+command:update() {
+    command:update-image
+    command:update-script
+}
+
+help:update() {
+    echo Pull the latest $FINAL_IMAGE, and then update $0 from that.
+}
+
+command:help() {
+    if [[ $# != 0 ]]; then
+        if ! has command $1; then
+            err \"$1\" is not an dockcross command
+            command:help
+        elif ! has help $1; then
+            err No help found for \"$1\"
+        else
+            help:$1
+        fi
+    else
+        cat >&2 <<ENDHELP
+usage: dockcross command [args]
+
+By default, runs the given command in an dockcross container.
+
+Builtin commands:
+    update-image
+    update-script
+    update
+
+For command help use: $0 help <command>
+ENDHELP
+    fi
+}
+
+#------------------------------------------------------------------------------
+# Option processing
+#
+while [[ $# != 0 ]]; do
+    case $1 in
+
+        --)
+            break
+            ;;
+
+        --args)
+            ARG_ARGS="$2"
+            shift 2
+            ;;
+
+        --config)
+            ARG_CONFIG="$2"
+            shift 2
+            ;;
+
+        --image)
+            ARG_IMAGE="$2"
+            shift 2
+            ;;
+
+        -*)
+            err Unknown option \"$1\"
+            command:help
+            exit
+            ;;
+
+        *)
+            break
+            ;;
+
+    esac
+done
+
+# The precedence for options is:
+# 1. command-line arguments
+# 2. environment variables
+# 3. defaults
+
+# Source the config file if it exists
+DEFAULT_CONFIG=~/.dockcross
+FINAL_CONFIG=${ARG_CONFIG-${DOCKCROSS_CONFIG-$DEFAULT_CONFIG}}
+
+[[ -f "$FINAL_CONFIG" ]] && source "$FINAL_CONFIG"
+
+# Set the docker image
+DEFAULT_IMAGE=thewtex/cross-compiler-base
+FINAL_IMAGE=${ARG_IMAGE-${DOCKCROSS_IMAGE-$DEFAULT_IMAGE}}
+
+# Set the docker run extra args (if any)
+FINAL_ARGS=${ARG_ARGS-${DOCKCROSS_ARGS}}
+
+# If we are not running via boot2docker
+if [ -z $DOCKER_HOST ]; then
+    USER_IDS="-e BUILDER_UID=$( id -u ) -e BUILDER_GID=$( id -g )"
+fi
+
+#------------------------------------------------------------------------------
+# Command-line processing
+#
+if [[ $# == 0 ]]; then
+    command:help
+    exit
+fi
+
+case $1 in
+
+    --)
+        # Everything after this is the command-line to be executed
+        shift
+        ;;
+
+    *)
+        # If this is a builtin command, execute it, otherwise fall through
+        if has command $1; then
+            command:$1 "${@:2}" # array slice skipping first element
+            exit $?
+        fi
+        ;;
+
+esac
+
+#------------------------------------------------------------------------------
+# Now, finally, run the command in a container
+#
+docker run -i -t --rm \
+    -v $PWD:/build \
+    $USER_IDS \
+    $FINAL_ARGS \
+    $FINAL_IMAGE "$@"
+
+################################################################################
+#
+#   This image is not intended to be run manually.
+#
+#   To install the dockcross helper, run the following commands:
+#
+#       docker run sdt4docker/raspberry-pi-cross-compiler > dockcross
+#       chmod +x dockcross
+#
+#   You may then wish to move dockcross to somewhere in your path.
+#
+################################################################################

imagefiles/entrypoint.sh

@@ -0,0 +1,30 @@
+#!/bin/bash
+
+# This is the entrypoint script for the dockerfile. Executed in the
+# container at runtime.
+
+if [[ $# == 0 ]]; then
+    # Presumably the image has been run directly, so help the user get started.
+    cat /dockcross/dockcross
+    exit 0
+fi
+
+# If we are running docker natively, we want to create a user in the container
+# with the same UID and GID as the user on the host machine, so that any files
+# created are owned by that user. Without this they are all owned by root.
+# If we are running from boot2docker, this is not necessary.
+# The dockcross script sets the BUILDER_UID and BUILDER_GID vars.
+if [[ -n $BUILDER_UID ]] && [[ -n $BUILDER_GID ]]; then
+
+    BUILDER_USER=dockcross-user
+    BUILDER_GROUP=dockcross-group
+
+    groupadd -o -g $BUILDER_GID $BUILDER_GROUP 2> /dev/null
+    useradd -o -g $BUILDER_GID -u $BUILDER_UID $BUILDER_USER 2> /dev/null
+
+    # Run the command as the specified user/group.
+    exec chpst -u :$BUILDER_UID:$BUILDER_GID "$@"
+else
+    # Just run the command as root.
+    exec "$@"
+fi