Add Cirrus CI (#7431)

* Cirrus: Add Tox task

* Cirrus: Add Locale task

* Cirrus: Add Flake8 Task

* Cirrus: Add Regtest task

* Regtest: Flush stdout

Allows viewing output sooner.

* Regtest: Read process.stdout in text mode

Improves ability to quickly see output.

* Cirrus: Add Windows task

* Cirrus: Add Android task

* Cirrus: Add macOS task

* Cirrus: Add AppImage task

* Cirrus: Add tarball task

* Cirrus: Add Submodules task

* Android: remove superfluous cp/rm

* Add .dockerignore

Symlink to .gitignore.

contrib/android/Dockerfile

@@ -151,7 +151,7 @@ RUN chown ${USER} /opt
 USER ${USER}
 
 
-COPY requirements-build-android.txt /opt/deterministic-build/
+COPY contrib/deterministic-build/requirements-build-android.txt /opt/deterministic-build/
 RUN python3 -m pip install --no-dependencies --user \
     -r /opt/deterministic-build/requirements-build-android.txt