add Dockerfile for automated build on docker cloud

.dockerignore

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+/.*
+build*

Dockerfile

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+
+ARG  CACHE_TAG=latest
+
+FROM  qgis/qgis3-build-deps:${CACHE_TAG}
+MAINTAINER Denis Rouzaud <denis.rouzaud@gmail.com>
+
+ENV CC=/usr/lib/ccache/clang
+ENV CXX=/usr/lib/ccache/clang++
+ENV QT_SELECT=5
+
+COPY  . /usr/src/QGIS
+
+WORKDIR /usr/src/QGIS/build
+
+RUN cmake \
+ -GNinja \
+ -DWITH_STAGED_PLUGINS=ON \
+ -DWITH_GRASS=ON \
+ -DSUPPRESS_QT_WARNINGS=ON \
+ -DENABLE_TESTS=OFF \
+ -DWITH_QSPATIALITE=ON \
+ -DWITH_QWTPOLAR=OFF \
+ -DWITH_APIDOC=OFF \
+ -DWITH_ASTYLE=OFF \
+ -DWITH_DESKTOP=ON \
+ -DWITH_BINDINGS=ON \
+ -DDISABLE_DEPRECATED=ON \
+ .. \
+ && ninja install \
+ && rm -rf /usr/src/QGIS/build/*