Refactor Dockerfile

Extracted patch application into dedicated RUN step so we end up with an
image right before patches are applied, which is conventient to debug.

Dockerfile

@@ -28,8 +28,9 @@ ARG KEYSTORE_PROPERTIES
 RUN curl -OL https://github.com/M66B/FairEmail/archive/$VERSION.tar.gz \
     && tar xvzf $VERSION.tar.gz \
     && cd FairEmail-$VERSION \
-    && (for patch in /patches/*; do patch -p0 < $patch; done) \
     && echo $KEYSTORE_FILE | base64 -d > keystore.jks \
-    && echo $KEYSTORE_PROPERTIES | base64 -d > keystore.properties \
-    && gradle --no-daemon assemble \
-    && cp app/build/outputs/apk/full/github/FairEmail-v$VERSION-full-github.apk /apk
+    && echo $KEYSTORE_PROPERTIES | base64 -d > keystore.properties
+RUN cd FairEmail-$VERSION \
+    && (for patch in /patches/*; do patch -p0 < $patch; done) \
+    && gradle --no-daemon assemble
+RUN cp FairEmail-$VERSION/app/build/outputs/apk/github/release/FairEmail-v$VERSION-github-release.apk /apk