over-simplistic instructions to building the Android package from source
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The easiest way to build things is using our container:

mkdir $HOME/src
cd $HOME/src
git clone https://github.com/subsurface/subsurface
cd subsurface
git checkout version or branch you are testing
cd ..
sudo docker run -v $HOME/src/subsurface:/android/subsurface -w /android --name=android-builder-docker -d subsurface/android-build-container:5.15.1 /bin/sleep 60m
sudo docker exec -t android-builder-docker git config --global user.email "ci@subsurface-divelog.org"
sudo docker exec -t android-builder-docker git config --global user.name "Subsurface CI"
sudo docker exec -t android-builder-docker apt-get install --reinstall cpp-7 gcc-7-base libgcc-7-dev libcc1-0 gcc-7
sudo docker exec -t android-builder-docker /bin/bash -x ./subsurface/packaging/android/qmake-build.sh


alternatively you can build locally without the help of our container.

Setup your build environment on a Ubuntu 20.04 Linux box

I think these packages should be enough:
sudo apt-get update
sudo apt-get install -y \
    autoconf \
    automake \
    cmake \
    git \
    libtool-bin \
    make \
    wget \
    unzip \
    python \
    python3-pip \
    bzip2 \
    pkg-config \
    libx11-xcb1 \
    libgl1-mesa-glx \
    libglib2.0-0 \
    openjdk-8-jdk \
    curl \
    coreutils \
    p7zip-full

sudo mkdir /android
sudo chown `id -un` /android
cd /android
wget https://dl.google.com/android/repository/commandlinetools-linux-6858069_latest.zip
unzip commandlinetools-linux-*.zip

git clone https://github.com/subsurface/subsurface

# now get the SDK, NDK, Qt, everything that's needed
bash /android/subsurface/packaging/android/android-build-setup.sh

Once this has completed, you should have a working build environment.

bash -x subsurface/packaging/android/qmake-build.sh

should build a working .aab as well as a .apk that can be installed on
your attached device:

./platform-tools/adb  install ./subsurface-mobile-build/android-build/build/outputs/apk/debug/android-build-debug.apk

Note that since you don't have the same signing key that I have,
you'll have to uninstalled the 'official' Subsurface-mobile binary in
order for this to work. And likewise you have to uninstall yours
before you'll be able to install an official binary again.