Add "Build Docker images (for arm on Raspberry Pi)"

Docker-usage-instructions.md

@@ -61,5 +61,33 @@ You can use these variables either with the `--env` Docker option or with a comb
 Please refer to the Docker documentation for more information:<br>
 * [Docker run - Set environment variables](https://docs.docker.com/engine/reference/commandline/run/#set-environment-variables--e---env---env-file)
 
+### Build Docker images (for arm on Raspberry Pi)
+If you use the Docker image on arm (for example Raspberry Pi), you'd build the image as below.
+
+```bash
+$ git clone https://github.com/Tyrrrz/DiscordChatExporter.git
+$ cd DiscordChatExporter
+$ docker build -t tyrrrz/discordchatexporter:latest .
+```
+
+After a while, some images are built like below.
+
+```bash
+$ docker image ls
+REPOSITORY                         TAG       IMAGE ID       CREATED              SIZE
+tyrrrz/discordchatexporter         latest    7c9ead725177   49 seconds ago       199MB
+<none>                             <none>    57f9f49d05af   About a minute ago   678MB
+mcr.microsoft.com/dotnet/sdk       5.0       72af6071941e   4 days ago           641MB
+mcr.microsoft.com/dotnet/runtime   3.1       6229bebd5a11   4 days ago           197MB
+```
+
+In these images, two images (`tyrrrz/discordchatexporter:latest` and `mcr.microsoft.com/dotnet/runtime:3.1`) are required so you may delete the other two images.
+
+Now you can execute DiscordChatExporter.
+
+```bash
+$ docker run --rm tyrrrz/discordchatexporter:latest
+```
+
 ##
 Special thanks to [@simnalamburt](https://github.com/simnalamburt) (dockerize) and [@Nimja](https://github.com/nimja) (better instructions)