feat: cleanup image, update base image

Dockerfile

+FROM openjdk:8-jdk as builder
+
+ARG PLATFORM=32
+ARG BUILD_TOOLS=32.0.0
+
+RUN apt-get --quiet update --yes && \
+    apt-get --quiet install --yes wget tar unzip lib32stdc++6 lib32z1
+RUN wget --quiet --output-document=android-sdk.zip https://dl.google.com/android/repository/sdk-tools-linux-4333796.zip && \
+    unzip -q android-sdk.zip -d android-sdk-linux && \
+    rm android-sdk.zip
+RUN mkdir android-sdk-linux/licenses && \
+    printf "8933bad161af4178b1185d1a37fbf41ea5269c55\nd56f5187479451eabf01fb78af6dfcb131a6481e\n24333f8a63b6825ea9c5514f83c2829b004d1fee" > android-sdk-linux/licenses/android-sdk-license && \
+    printf "84831b9409646a918e30573bab4c9c91346d8abd" > android-sdk-linux/licenses/android-sdk-preview-license
+RUN android-sdk-linux/tools/bin/sdkmanager --update > /dev/null
+RUN android-sdk-linux/tools/bin/sdkmanager \
+      "build-tools;${BUILD_TOOLS}" \
+      > /dev/null
+
 FROM ubuntu:focal
+COPY --from=builder /android-sdk-linux /android-sdk-linux
 
 ADD src/fdroid.list /etc/apt/sources.list.d/fdroid.list
 ADD src/fdroid.asc /usr/share/keyrings/fdroid.asc
 RUN apt-get update && \
-    DEBIAN_FRONTEND=noninteractive TZ=Etc/UTC apt-get -y install --no-install-recommends python3-pip python3-wheel python3-setuptools apksigner fdroidserver && \
+    DEBIAN_FRONTEND=noninteractive TZ=Etc/UTC apt-get -y install --no-install-recommends python3-pip python3-wheel python3-setuptools fdroidserver && \
     rm -rf /var/lib/apt/lists/* && \
     pip3 install s3cmd
 
-RUN unlink /usr/bin/apksigner
-ADD src/apksigner.sh /usr/bin/apksigner
 ADD src/update_repo.sh /
 
 ENV UPDATE_WAIT=10