Dockerfiles and Docker Images
A Dockerfile is a text document containing instructions to build Docker images. These images consist of read-only layers, each layer corresponding to a Dockerfile instruction. Docker can automatically create images by interpreting these instructions. The following command is used to build an image from a Dockerfile:
docker build -f <dockerfile_path>
Dockerfile Instructions
1. FROM
The FROM instruction initializes a new build stage and sets the base image for subsequent instructions. A valid Dockerfile must begin with a FROM instruction. The base image can be any valid image.
Syntax:
FROM <Image_name>:<Image_tag>
2. LABEL
The LABEL instruction adds metadata to an image, helping to organize images by project or record licensing information. Each label is defined as a key-value pair.
Examples:
LABEL com.example.version="0.0.1-beta"
LABEL vendor1="ACME Incorporated"
3. RUN
The RUN instruction executes commands in a new layer on top of the current image and commits the results. The resulting image serves as the base for the next step.
Example:
FROM ubuntu:14.04
RUN apt-get update
RUN apt-get install -y curl
4. CMD
The CMD instruction specifies the default command to run in the container. Only one CMD instruction is allowed per Dockerfile; the last CMD overrides any previous ones.
Syntax:
CMD ["executable", "param1", "param2"]
5. EXPOSE
The EXPOSE instruction specifies the ports on which the container listens for connections.
Syntax:
EXPOSE <port>
6. ENV
The ENV instruction sets environment variables for the container. It can also update the PATH variable to make software easier to run.
Example:
ENV PATH /usr/local/nginx/bin:$PATH
7. ADD
The ADD instruction copies files, directories, or remote URLs from the source to the specified destination within the image.
Example:
ADD hom* /mydir/ # Adds all files starting with “hom”
8. VOLUME
The VOLUME instruction designates storage areas, such as database files or configuration storage, that should be persisted outside the container.
9. WORKDIR
The WORKDIR instruction sets the working directory for subsequent RUN, CMD, and ADD instructions.
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