The configuration file. Sets up a Docker container when you run docker build on it. Vastly preferable to docker commit.

Here are some common text editors and their syntax highlighting modules you could use to create Dockerfiles:

If you use jEdit, I've put up a syntax highlighting module for Dockerfile you can use.
Sublime Text 2
Atom
Vim
Emacs
TextMate
VS Code
Also see Docker meets the IDE

Instructions

.dockerignore
FROM Sets the Base Image for subsequent instructions.
MAINTAINER (deprecated - use LABEL instead) Set the Author field of the generated images.
RUN execute any commands in a new layer on top of the current image and commit the results.
CMD provide defaults for an executing container.
EXPOSE informs Docker that the container listens on the specified network ports at runtime. NOTE: does not actually make ports accessible.
ENV sets environment variable.
ADD copies new files, directories or remote file to container. Invalidates caches. Avoid ADD and use COPY instead.
COPY copies new files or directories to container. By default this copies as root regardless of the USER/WORKDIR settings. Use --chown=<user>:<group> to give ownership to another user/group. (Same for ADD.)
ENTRYPOINT configures a container that will run as an executable.
VOLUME creates a mount point for externally mounted volumes or other containers.
USER sets the user name for following RUN / CMD / ENTRYPOINT commands.
WORKDIR sets the working directory.
ARG defines a build-time variable.
ONBUILD adds a trigger instruction when the image is used as the base for another build.
STOPSIGNAL sets the system call signal that will be sent to the container to exit.
LABEL apply key/value metadata to your images, containers, or daemons.
SHELL override default shell is used by docker to run commands.
HEALTHCHECK tells docker how to test a container to check that it is still working.

Tutorial