add dockerfile for building appimage in a container

Dockerfile

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+FROM ubuntu:22.04
+
+# install deps
+RUN apt-get update && apt-get install -y --no-install-recommends file ca-certificates wget git pkg-config libasound2 libjack-jackd2-dev libasound2-dev libssl-dev libxcb-render0-dev libxcb-shape0-dev libxcb-xfixes0-dev libxkbcommon-dev libgtk-3-0 libgtk-3-dev curl build-essential
+
+# fix cert
+RUN update-ca-certificates
+
+# install rust
+RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y
+
+# configure shell
+ENV PATH="/root/.cargo/bin:${PATH}"
+
+# install cargo-appimage
+RUN cargo install cargo-appimage # file package is required by appimagetool
+
+# switch to nightly
+RUN rustup default nightly
+
+# latest version
+RUN rustup update
+
+# download appimagetool
+RUN wget https://github.com/AppImage/AppImageKit/releases/download/continuous/appimagetool-$(uname -m).AppImage -O /usr/local/bin/appimagetool
+
+# apply hacks
+RUN chmod +x /usr/local/bin/appimagetool # Path appimagetool magic byte: https://github.com/AppImage/pkg2appimage/issues/373#issuecomment-495754112
+
+# apply hacks
+RUN sed -i 's|AI\x02|\x00\x00\x00|' /usr/local/bin/appimagetool # Use appimagetool without fuse: https://github.com/AppImage/AppImageKit/wiki/FUSE#docker
+
+# get megra
+RUN git clone https://github.com/the-drunk-coder/megra.rs
+
+# latest stuff
+RUN cd megra.rs && cargo update
+
+# build appimage
+RUN cd megra.rs && APPIMAGE_EXTRACT_AND_RUN=1 cargo appimage