container.training/slides/containers/More_Dockerfile_Instructions.md

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More Dockerfile Instructions

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Dockerfile usage summary

• Dockerfile instructions are executed in order.

• Each instruction creates a new layer in the image.

• Docker maintains a cache with the layers of previous builds.

• When there are no changes in the instructions and files making a layer, the builder re-uses the cached layer, without executing the instruction for that layer.

• The FROM instruction MUST be the first non-comment instruction.

• Lines starting with # are treated as comments.

• Some instructions (like CMD or ENTRYPOINT) update a piece of metadata.

  (As a result, each call to these instructions makes the previous one useless.)

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The EXPOSE instruction

The EXPOSE instruction tells Docker what ports are to be published in this image.

EXPOSE 8080
EXPOSE 80 443
EXPOSE 53/tcp 53/udp

• All ports are private by default.

• Declaring a port with EXPOSE is not enough to make it public.

• The Dockerfile doesn't control on which port a service gets exposed.

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Exposing ports

• When you docker run -p <port> ..., that port becomes public.

  (Even if it was not declared with EXPOSE.)

• When you docker run -P ... (without port number), all ports declared with EXPOSE become public.

A public port is reachable from other containers and from outside the host.

A private port is not reachable from outside.

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VOLUME

The VOLUME instruction tells Docker that a specific directory should be a volume.

VOLUME /var/lib/mysql

Filesystem access in volumes bypasses the copy-on-write layer, offering native performance to I/O done in those directories.

Volumes can be attached to multiple containers, allowing to "port" data over from a container to another, e.g. to upgrade a database to a newer version.

It is possible to start a container in "read-only" mode. The container filesystem will be made read-only, but volumes can still have read/write access if necessary.

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The WORKDIR instruction

The WORKDIR instruction sets the working directory for subsequent instructions.

It also affects CMD and ENTRYPOINT, since it sets the working directory used when starting the container.

WORKDIR /src

You can specify WORKDIR again to change the working directory for further operations.

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The ENV instruction

The ENV instruction specifies environment variables that should be set in any container launched from the image.

ENV WEBAPP_PORT 8080

This will result in an environment variable being created in any containers created from this image of

WEBAPP_PORT=8080

You can also specify environment variables when you use docker run.

$ docker run -e WEBAPP_PORT=8000 -e WEBAPP_HOST=www.example.com ...

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The USER instruction

The USER instruction sets the user name or UID to use when running the image.

It can be used multiple times to change back to root or to another user.

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:EN:- Advanced Dockerfile syntax :FR:- Dockerfile niveau expert