Add note about install python on alpine (#6677)

When not using a python base image and using alpine, you need to install
python by yourself. You should also pin the python version when doing
so; currently, i see only python 3.12 in the alpine repository.

docs/guides/integration/docker.md

@@ -141,6 +141,17 @@ $ docker run -it $(docker build -q .) /bin/bash -c "cowsay -t hello"
     ENV UV_TOOL_BIN_DIR=/opt/uv-bin/
     ```
 
+### Installing Python in musl-based images
+
+While uv [installs a compatible Python version](../install-python.md) if there isn't one available
+in the image, uv does not yet support installing Python for musl-based distributions. For example,
+if you are using an Alpine Linux base image that doesn't have Python installed, you need to add it
+with the system package manager:
+
+```shell
+apk add --no-cache python3~=3.12
+```
+
 ## Developing in a container
 
 When developing, it's useful to mount the project directory into a container. With this setup,