Finally works!

Dockerfile

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+FROM ubuntu:20.04
+LABEL [email protected]
+
+ARG DEBIAN_FRONTEND=noninteractive
+RUN apt -y update
+
+RUN apt -y install \
+           flatpak \
+           flatpak-builder && \
+    rm -rf /var/cache/apt/archives/*
+
+RUN flatpak remote-add --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo
+
+COPY docker-entrypoint.sh /docker-entrypoint.sh
+RUN chmod +x /docker-entrypoint.sh
+
+ENTRYPOINT ["/docker-entrypoint.sh"]

README.md

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+Flatpak repository for Firewall Builder
+==============================================
+
+Create a Flatpak repository for Firewall Builder https://github.com/fwbuilder/fwbuilder. Usage of container is not necessary but provides clean flatpak-builder environment.
+
+First clone repository:
+
+`git clone https://github.com/HenriWahl/com.github.fwbuilder.git`
+
+Change into cloned repository folder:
+
+`cd com.github.fwbuilder`
+
+Build container as clean flatpak-builder environment:
+
+`docker build -t flatpak_builder .`
+
+Run flatpak-builder inside privileged container - don't forget to replace volume source to you cloned git repo:
+
+`docker run -it --rm -v $PWD/com.github.fwbuilder:/build --privileged flatpak_builder:latest`
+
+Add freshly built local flatpak repository:
+
+`flatpak remote-add --no-gpg-verify fwbuilder_repo fwbuilder_repo`
+
+Install from flatpak repository:
+
+`flatpak install fwbuilder_repo com.github.fwbuilder`
+
+Run Firewall Builder:
+
+`flatpak run com.github.fwbuilder`

com.github.fwbuilder.yaml

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+id: com.github.fwbuilder
+runtime: org.kde.Platform
+runtime-version: '5.14'
+sdk: org.kde.Sdk
+rename-desktop-file: fwbuilder.desktop
+rename-icon: fwbuilder
+command: fwbuilder
+cleanup:
+- /include
+- /lib/pkgconfig
+- /share/doc
+- /share/man
+- '*.a'
+- '*.la'
+finish-args:
+- --device=dri
+- --filesystem=home
+- --share=ipc
+- --share=network
+- --socket=fallback-x11
+- --socket=wayland
+- --socket=x11
+modules:
+- build-options:
+    ldflags: -lncurses -ltinfo
+  builddir: true
+  buildsystem: autotools
+  name: net-snmp
+  sources:
+  - commit: 9e933893751fb5b1b68c6efb844ef5d4b3a905e0
+    type: git
+    url: https://github.com/net-snmp/net-snmp
+- builddir: true
+  buildsystem: cmake-ninja
+  config-opts:
+  - -DNETSNMP_INCLUDE_DIR=/app/include
+  name: fwbuilder
+  sources:
+  - commit: ea25d1e557d45d10d4354cf1435633b5d37edf3d
+    type: git
+    url: https://github.com/fwbuilder/fwbuilder
+

docker-entrypoint.sh

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+#!/bin/sh
+
+# install needed frameworks at runetime because not possible to build dockerfile as privileged user
+flatpak install --assumeyes org.kde.Platform/x86_64/5.14 \
+                            org.kde.Sdk/x86_64/5.14
+
+# mounted git repo clone
+cd /build
+
+# build with local repo folder to be later used for installation
+flatpak-builder --repo=fwbuilder_repo fwbuilder_build --force-clean com.github.fwbuilder.yaml