Docker: bump Qt to v5.15.3

Dockerfile.linux

@@ -73,7 +73,7 @@ RUN git clone -b 2.13.92 --depth 1 https://gitlab.freedesktop.org/fontconfig/fon
     make -j$THREADS install && \
     rm -rf $(pwd)
 
-ENV QT_VERSION=v5.15.2
+ENV QT_VERSION=v5.15.3-lts-lgpl
 RUN git clone git://code.qt.io/qt/qt5.git -b ${QT_VERSION} --depth 1 && \
     cd qt5 && \
     git clone git://code.qt.io/qt/qtbase.git -b ${QT_VERSION} --depth 1 && \

Dockerfile.windows

@@ -1,7 +1,7 @@
 FROM ubuntu:20.04
 
 ARG THREADS=1
-ARG QT_VERSION=5.15.2
+ARG QT_VERSION=v5.15.3-lts-lgpl
 ENV SOURCE_DATE_EPOCH=1397818193
 ENV DEBIAN_FRONTEND=noninteractive