feat(docker): add .NET installation directory for permission handling

- Sets `DOTNET_INSTALL_DIR` to `/home/runner/.dotnet` in the Docker image to avoid permission issues when using actions/setup-dotnet.
- Updates README to document the new environment variable for user reference.

Author: winromulus

README.md

@@ -82,6 +82,10 @@ docker run -d \
 - `GITHUB_RUNNER_GID` (optional): Custom GID to create github-actions-runner group
 - `GITHUB_RUNNER_DOCKER_SOCK` (optional): Set to "true" to auto-configure Docker socket access
 
+##### Pre-configured Environment Variables
+The following environment variables are set in the Docker image:
+- `DOTNET_INSTALL_DIR`: Set to `/home/runner/.dotnet` to avoid permission issues when using actions/setup-dotnet
+
 ## Included Software
 
 ### Pre-installed in Base Image

src/Dockerfile

@@ -35,5 +35,8 @@ USER runner
 # Set working directory
 WORKDIR /home/runner
 
+# Set .NET install directory to avoid permission issues
+ENV DOTNET_INSTALL_DIR=/home/runner/.dotnet
+
 # Set entrypoint
 ENTRYPOINT ["/entrypoint.sh"]