Dockerfile instruction

Instruction is information on how to build docker image.

FROM

FROM image:tag AS alias FROM node:14 AS build-image

• FROM initializes a new build stage and base image for subsequent instructions. Images can be pull from - public or private repository.
• FROM can appear mutiple times to used for make a build stage that can be copy and use on the final stage.

WORKDIR

WORKDIR part WORKDIR /path/to/workdir

• This set the working directory inside docker(also create directory). This will affect the rest instruction that refer to directory.

COPY

COPY . /app COPY source_dir destination_dir

• This copy file from outside of docker into inside

ADD

APP . /app APP source_dir destination_dir

• This is extended from COPY. There is 2 other ways to import to docker that copy way.

ENV

ENV endpoint=http://loaclhost:8080 ENV key=value

• This set environment variable inside docker to be the given value.

ARG

ARG key=default-value ARG pet=cat

• This defind the variable that user can pass when exeucte “docker build” and we can also defind the default value to be used in case that user omitted the value.

RUN

RUN apt-get update && apt-get install git RUN command (shell form) RUN [“executable”, “param1”, “param2”] (exec form)

• RUN usually used for install package required for application to run.

CMD

CMD [“echo”, “Hello World”] CMD [“executable”, “param1”, “param2”] (exec form) CMD echo “Hello World” CMD command param1 param2 (shell form)

• CMD main purpose is to provides default for an executting container. CMD can be overriden by arguments when execute “docker run”.

ENTRYPOINT

ENTRYPOINT [“echo”, “Hello World”] ENTRYPOINT [“executable”, “param1”, “param2”] (exec form) ENTRYPOINT echo “Hello World” ENTRYPOINT command param1 param2 (shell form)

• This works similar to CMD but the difference is that command and params aren’t ignored when run container with passing command and params. ENTRYPOINT and CMD can work together as ENTRYPOINT-> should execute anyway command and CMD -> command default args.

EXPOSE

EXPOSE 8080 EXPOSE port

• This will expose the specified port on container with TCP as default protocal. User can use this port later when run container to forwards request into container.