To install a PHP extension in Docker, you can use the `docker-php-ext-install` command. Here's an example of how you can install the `gd` extension:

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1. Create a Dockerfile in your project directory.

2. Use a base PHP image that matches the version you want to use. For example, if you want to use PHP 7.4, you can use the `php:7.4-fpm` image.

3. Add the following lines to your Dockerfile:

​

```Dockerfile

# Install the gd extension

RUN apt-get update && \

apt-get install -y libpng-dev && \

docker-php-ext-install gd

```

​

In this example, we're using the `libpng-dev` package as a dependency for installing the `gd` extension. You might need to install other dependencies depending on the extension you want to install.

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4. Run the `docker build` command to build your Docker image:

​

```bash

docker build -t my-php-app .

```

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Replace `my-php-app` with the desired name for your Docker image.

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5. Once the image is built, you can run a container based on it:

​

```bash

docker run -d -p 8080:80 my-php-app

```

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Replace `8080` with the desired port for your container.

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By following these steps, you should be able to install PHP extensions within a Docker container using the `docker-php-ext-install` command. Remember to adjust the commands and dependencies according to the specific extension you want to