Fix remaining MIT license references

- Update README.md license section from MIT to Apache 2.0
- Update Dockerfile.multiarch license label from MIT to Apache-2.0

Dockerfile.multiarch

@@ -69,7 +69,7 @@ LABEL org.opencontainers.image.title="HMAC File Server" \
       org.opencontainers.image.version="3.3.0" \
       org.opencontainers.image.vendor="UUXO" \
       org.opencontainers.image.source="https://git.uuxo.net/uuxo/hmac-file-server/" \
-      org.opencontainers.image.licenses="MIT" \
+      org.opencontainers.image.licenses="Apache-2.0" \
       org.opencontainers.image.architecture="multi"
 
 # Entry point

README.md

@@ -2343,7 +2343,7 @@ curl http://localhost:9090/metrics
 
 ## License
 
-This project is licensed under the MIT License - see the [LICENSE](LICENSE) file for details.
+This project is licensed under the Apache License 2.0 - see the [LICENSE](LICENSE) file for details.
 
 ---