DockerFile
Every Docker container starts with a simple text file containing instructions for how to build the Docker container image. DockerFile automates the process of Docker image creation. It’s essentially a list of commands that Docker Engine will run in order to assemble the image.
To create a new image we need a "Dockerfile". There are a dozen instructions we can use in our Dockerfile.
FROM - This instruction is used to set the base image for subsequent instructions. It is mandatory to have it in the first-line and we can use it any number of times.
MAINTAINER - Used to indicate the author of the image.
RUN - We can run any command on top of an existing layer and create a new layer with the results of command execution.
CMD - The major difference between CMD and RUN is that CMD doesn't execute anything during the build time. It just specifies the intended command for the image. Whereas RUN executes the command during image build time.
Note: There can be only one CMD instruction in a Dockerfile, if you add more, only the last one takes effect.
EXPOSE - While running your service in the container you may want your container to listen on specified ports. The EXPOSE instruction helps you do this.
ENV - This instruction can be used to set the environment variables in the container.
COPY - This instruction is used to copy files and directories from a specified source to a destination.
ADD - This instruction is similar to COPY but with a few added features like remote URL support in the source field and local-only tar extraction. But if you don't need an extra feature, it is suggested to use COPY as it is more readable.
ENTRYPOINT - This instruction is to set the primary command for the image.
VOLUME - you can use the VOLUME instruction to enable access to a location on the host system from a container. Just pass the path to the location to be accessed.
USER - This is to set the UID (or username) to use when running the image.
WORKDIR - To set the currently active directory for other instructions such as RUN, CMD, ENTRYPOINT, COPY, and ADD. Note that if a relative path is provided, the next WORKDIR instruction will take it as relative to the path of the previous WORKDIR.
ONBUILD - Adds a trigger instruction to be executed when the image is used as the base for some other image. It behaves as if a RUN instruction is inserted immediately after the FROM instruction of the downstream Dockerfile.
This is typically helpful in cases where you need a static base image with a dynamic config value that changes whenever a new image must be built.
To build an image from a Dockerfile use the below command.
$ sudo docker image build -t $account_name/$image_name:$tag $path_of_DockerfileExample1: Dockerfile
FROM ubuntu:latest
MAINTAINER nikhil555
ENV ENV=Test
RUN apt update && apt install wget zip curl nginx -y && echo "Installed wget, zip, curl, nginx"
RUN echo "Nginx is installed"
EXPOSE 80 443
CMD [ "nginx", "-g", "daemon off;" ]
The above Dockerfile creates a custom image from the Ubuntu base image, and installs wget, zip, curl and Nginx packages on it and starts the nginx service when the container is created from this image.
Example2: Dockerfile
FROM nginx:latest
WORKDIR /usr/share/nginx/html
COPY index.html index.html
"index.html" File contents:
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Your Dockerfile worked!</title>
</head>
<body>
<h1>You just successfully ran a container with a custom file copied into the image at build time!</h1>
</body>
</html>
It is highly recommended you always pin the version of the image to be used. In this dockerfile we are using nginx official image and replacing the index.html file. We don't have to specify EXPOSE or CMD because they're in my FROM (i.e. in the base image dockerfile).
Note: using WORKDIR is preferred to using 'RUN cd /some/path'
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