I am experimenting with building my own Docker image with the purpose of later building Jenkins and SonarQube dockers.

When I attempt to create a simple Dockerfile to experiment and install emacs to easily be able to edit files in bash I get the following output:

…
Setting up colord (1.0.6-1) ...
Setting up gconf-service (3.2.6-0ubuntu2) ...
Setting up emacs24 (24.3+1-2ubuntu1) ...
update-alternatives: using /usr/bin/emacs24-x to provide /usr/bin/emacs (emacs) in auto mode
Install emacsen-common for emacs24
emacsen-common: Handling install of emacsen flavor emacs24
Wrote /etc/emacs24/site-start.d/00debian-vars.elc
Wrote /usr/share/emacs24/site-lisp/debian-startup.elc
Setting up emacs (45.0ubuntu1) ...
Setting up gconf-service-backend (3.2.6-0ubuntu2) ...
Processing triggers for libc-bin (2.19-0ubuntu6.6) ...
Processing triggers for sgml-base (1.26+nmu4ubuntu1) ...
Processing triggers for libgdk-pixbuf2.0-0:amd64 (2.30.7-0ubuntu1.2) ...
---> 91050783b5ea
Removing intermediate container 38a6c797bbdb
Step 4 : CMD /usr/games/fortune -a | cowsay
---> Running in 1dd25d8bcf95
---> a874940ac99e
Removing intermediate container 1dd25d8bcf95
Successfully built a874940ac99e
SECURITY WARNING: You are building a Docker image from Windows against a non-Windows Docker host. All files and directories added to build context will have '-rwxr-xr-x' permissions. It is recommended to double check and reset permissions for sensitive files and directories.

This is what my dockerfile contains:

FROM docker/whalesay:latest

RUN apt-get -y update && apt-get install -y fortunes
RUN apt-get update && apt-get install -y emacs

CMD /usr/games/fortune -a | cowsay

What does this Security warning mean exactly and how can I avoid it?

More info, when I use the docker info command I get the following data:

$ docker info
Containers: 4
Running: 1
Paused: 0
Stopped: 3
Images: 6
Server Version: 1.10.1
Storage Driver: aufs
Root Dir: /mnt/sda1/var/lib/docker/aufs
Backing Filesystem: extfs
Dirs: 55
Dirperm1 Supported: true
Execution Driver: native-0.2
Logging Driver: json-file
Plugins:
Volume: local
Network: bridge null host
Kernel Version: 4.1.17-boot2docker
Operating System: Boot2Docker 1.10.1 (TCL 6.4.1); master : b03e158 - Thu Feb 11 22:34:01 UTC 201
OSType: linux
Architecture: x86_64
CPUs: 1
Total Memory: 1.956 GiB
Name: default
ID: NKSE:6GZL:N7AE:47Q5:HBAF:CERC:MHWZ:HWEC:QSVI:FTK5:P2E2:HRM3
Debug mode (server): true
File Descriptors: 27
Goroutines: 44
System Time: 2016-02-17T13:57:00.11534859Z
EventsListeners: 1
Init SHA1:
Init Path: /usr/local/bin/docker
Docker Root Dir: /mnt/sda1/var/lib/docker
Labels:
provider=virtualbox