Fix Docker build (#570)

scripts/Dockerfile

@@ -30,7 +30,7 @@ RUN apk add --no-cache --virtual .build-tools git g++ build-base linux-headers c
     cd .. && \
     git clone https://github.com/ToruNiina/toml11 --depth=1 && \
     cd toml11 && \
-    cmake . && \
+    cmake -DCMAKE_CXX_STANDARD=11 . && \
     make install -j $THREADS && \
     cd .. && \
     git clone https://github.com/tindy2013/subconverter --depth=1 && \