[docker] use ubuntu 16.04 and deal with user/group ids != 1000

- Update to Ubuntu xenial and switch to new gcc-arm-embedded PPA.
- Hhandle permissions for docker volumes by dynamically changing the id of user pprz to LOCAL_USER_ID.
This should finally make it possible use the shared volume with the paparazzi source in docker on machines where your user/group id is not 1000

docker/dev/entrypoint.sh

@@ -0,0 +1,26 @@
+#!/bin/bash
+
+# If user exists, change user and group ids, otherwise add new local user
+# Either use the LOCAL_USER_ID if passed in at runtime or
+# fallback
+
+USER_NAME=${USERNAME:-pprz}
+USER_ID=${LOCAL_USER_ID:-1000}
+GROUP_ID=${LOCAL_GROUP_ID:-1000}
+
+if id -u $USER_NAME > /dev/null 2>&1; then
+    echo "Starting with UID: $USER_ID and GID: $GROUP_ID"
+    # modify existing pprz user instead of creating new one
+    #usermod -o --uid $USER_ID $USER_NAME
+    groupmod -o --gid $GROUP_ID $USER_NAME
+    # usermod goes through all files in home dir and changes the userid, which takes a while
+    # since we don't want/need that here, directly change the user id in /etc/passwd
+    sed -i "s/$USER_NAME:x:\([0-9]*\):\([0-9]*\):/$USER_NAME:x:$USER_ID:\2:/g" /etc/passwd
+else
+    echo "Adding new user $USER_NAME with UID: $USER_ID and GID: $GROUP_ID"
+    useradd --shell /bin/bash -u $USER_ID -o -c "" -m $USER_NAME
+fi
+
+export HOME=/home/$USER_NAME
+
+exec /usr/local/bin/gosu $USER_NAME "$@"