Dockerfile Instructions: Base Image, Environment Setup, Comments, Installing Dependencies, and Copying Files

> the syntax before building, making sure you are using the most current version. ### Base image The line following the syntax directive defines what base image to use: ```dockerfile FROM ubuntu:22.04 ``` The [`FROM` instruction](/reference/dockerfile.md#from) sets your base image to the 22.04 release of Ubuntu. All instructions that follow are executed in this base image: an Ubuntu environment. The notation `ubuntu:22.04`, follows the `name:tag` standard for naming Docker images. When you build images, you use this notation to name your images. There are many public images you can leverage in your projects, by importing them into your build steps using the Dockerfile `FROM` instruction. [Docker Hub](https://hub.docker.com/search?image_filter=official&q=&type=image) contains a large set of official images that you can use for this purpose. ### Environment setup The following line executes a build command inside the base image. ```dockerfile # install app dependencies RUN apt-get update && apt-get install -y python3 python3-pip ``` This [`RUN` instruction](/reference/dockerfile.md#run) executes a shell in Ubuntu that updates the APT package index and installs Python tools in the container. ### Comments Note the `# install app dependencies` line. This is a comment. Comments in Dockerfiles begin with the `#` symbol. As your Dockerfile evolves, comments can be instrumental to document how your Dockerfile works for any future readers and editors of the file, including your future self! > [!NOTE] > > You might've noticed that comments are denoted using the same symbol as the > [syntax directive](#dockerfile-syntax) on the first line of the file. > The symbol is only interpreted as a directive if the pattern matches a > directive and appears at the beginning of the Dockerfile. Otherwise, it's > treated as a comment. ### Installing dependencies The second `RUN` instruction installs the `flask` dependency required by the Python application. ```dockerfile RUN pip install flask==3.0.* ``` A prerequisite for this instruction is that `pip` is installed into the build container. The first `RUN` command installs `pip`, which ensures that we can use the command to install the flask web framework. ### Copying files The next instruction uses the [`COPY` instruction](/reference/dockerfile.md#copy) to copy the `hello.py` file from the local build context into the root directory of our image. ```dockerfile COPY hello.py / ``` A [build context](./context.md) is the set of files that you can access in Dockerfile instructions such as `COPY` and `ADD`. After the `COPY` instruction, the `hello.py` file is added to the filesystem

This section details key Dockerfile instructions, starting with the `FROM` instruction to specify the base image (e.g., Ubuntu 22.04). It explains how to set up the environment using the `RUN` instruction to execute commands within the base image, such as installing Python tools. The use of comments for documentation is highlighted. It describes how to install application dependencies using `pip` and how to copy files from the local build context into the image using the `COPY` instruction.