Using ccache in automated builds on Docker cloud

I am using automated builds on Docker cloud to compile a C++ app and provide it in an image. Compilation is quite long (range 2-3 hours) and commits on github are frequent (~10 to 30 per day).

Is there a way to keep the building cache (using ccache) somehow?

As far as I understand it, docker caching is useless since the compilation layer producing the ccache will not be used due to the source code changes. Or can we tweak to bring some data back to first layer?

Any other solution? Pushing it somewhere?

Here is the Dockerfile:

# CACHE_TAG is provided by Docker cloud
# see https://docs.docker.com/docker-cloud/builds/advanced/
# using ARG in FROM requires min v17.05.0-ce
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 \
 -DCMAKE_INSTALL_PREFIX=/usr \
 -DBINDINGS_GLOBAL_INSTALL=ON \
 -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

WORKDIR /