The Dockerfile describes the steps used to create a Build from your application code.

    FROM ubuntu:18.04
    COPY . .
    RUN ["deps", "install"]
    CMD ["bin/start"]

Common Directives

Directive Description
FROM defines the base image
COPY add files from the local directory tree
RUN execute a command
CMD defines the default command to run on this image
ARG define build variables

Optimizing Build Times

Each line of a Dockerfile will be cached as long as files referenced by it are not changed. This allows you to cache expensive steps such as dependency installation by selectively copying files before running commands.

The following example selectively copies only the files needed to run npm before installing dependencies.

    FROM nodejs

    COPY package.json package-lock.json .
    RUN ["npm", "install"]

    COPY . .
    CMD ["npm", "start"]

The npm install will be cached on successive builds unless package.json or package-lock.json is changed.

Build Variables

Convox respects the ARG directive, allowing you to specify variables at build time.

This is useful for creating dynamic build environments, allowing you to use the same Dockerfile for varying deployments.

It is not recommended to use build variables for passing secrets. Values for build variables are embedded in the resulting image.

You can declare build variables using the ARG directive with an optional default value:


Values for these variables will be read from the Environment at build time:

    $ convox env set RUBY_VERSION=2.6.4

See Also