DOCKERFILE is a text file that has instructions to build a Docker image. All commands in dockerfile could also be used from the command line to build images.

docker build command creates an image from the dockerfile and its CONTEXTS. Contexts are a set of files at a PATH/URL. URL could be any repository location of any version control system. PATH could also be a location in your local system. The following command shows building docker image from the contexts in the current directory and send the context to the docker daemon

Root directory or path is not preferred as context path because the entire contents of your hard drive will be transferred to the Docker daemon.

Docker daemon performs a validation of the dockerfile before running instructions in dockerfile. Docker daemon takes on the instructions one by one creating an updated image each time.

Sample Dockerfile :

Each instruction in a dockerfile  creates one read-only layer:

• FROM creates a layer from the ubuntu:16.04 Docker image as the base image.
• COPY adds files or contexts from your Docker client’s current directory.
• RUN builds your application. Here we use make.
• CMD specifies the command to be run INSIDE the container.