Dockerfile

Dockerfile is a text document that containes all the commands a user could call on the command line to assemble an image. Using docker build users can create ana automated build that executes several command-line instructions in succession.

Whenever possible, Docker will reuse the intermediate images(cache), to accelerate the docker build process significantly.

Here is the format of the Dockerfile:
# Comment
INSTRUCTION arguments

A Dockerfile must start with a "FROM" instruction

FROM  -> Used to specify the base image from your custome image
USER   -> User that will run the container
ADD    -> Copy some file in your current directory into some dir inside the image
EXPOSE -> informs Docker that the container listens on the specified network ports at runtime
RUN  -> run some commands in the container
CMD -> command that will be executed when the container started
WORKDIR -> The working directory of the running container
ENV -> Inject some environment variable inside the container

Example:
FROM centos:latest
RUN yum -y install epel-release && yum -y install nginx && yum clean all
ADD index.html /usr/share/nginx/html/
EXPOSE 80 443
RUN systemctl enable nginx
CMD ["/usr/sbin/init"]

These are written in a file named "Dockerfile"


Build the image
# docker build -t custom-nginx .
.  -> means current directory
t --> means "allocate a terminal"

Run the image
# docker run -d --name nginx -p 8080:80 custom-nginx
8080:80 -> forward port 8080 to 80


Here is one example.