Add ARM64 (aarch64) support for AppImage builds

This commit enables building AppImages for both x86_64 and aarch64
architectures, making Pallectrum accessible on ARM-based systems

contrib/build-linux/appimage/Dockerfile

@@ -80,9 +80,11 @@ ARG UID=1000
 RUN if [ "$UID" != "0" ] ; then useradd --uid $UID --create-home --shell /bin/bash "user" ; fi
 RUN usermod -append --groups sudo $(id -nu $UID || echo "user")
 RUN echo "%sudo ALL=(ALL) NOPASSWD: ALL" >> /etc/sudoers
-RUN HOME_DIR=$(getent passwd $UID | cut -d: -f6)
+RUN HOME_DIR=$(getent passwd $UID | cut -d: -f6) && \
-ENV WORK_DIR="${HOME_DIR}/wspace" \
+    echo "WORK_DIR=${HOME_DIR}/wspace" >> /etc/environment && \
-    PATH="${HOME_DIR}/.local/bin:${PATH}"
+    echo "PATH=${HOME_DIR}/.local/bin:${PATH}" >> /etc/environment
+ENV WORK_DIR="/home/user/wspace" \
+    PATH="/home/user/.local/bin:${PATH}"
 WORKDIR ${WORK_DIR}
 RUN chown --recursive ${UID} ${WORK_DIR}
 USER ${UID}

contrib/build-linux/appimage/README.md

@@ -4,14 +4,16 @@ AppImage binary for Electrum
 ✓ _This binary should be reproducible, meaning you should be able to generate
    binaries that match the official releases._
 
-- _Minimum supported target system (i.e. what end-users need): x86_64, glibc 2.31_
+- _Minimum supported target system (i.e. what end-users need):_
+  - _x86_64: glibc 2.31_
+  - _aarch64 (ARM64): glibc 2.31_
 
-This assumes an Ubuntu host, but it should not be too hard to adapt to another
+This assumes an Ubuntu (or similar Debian-based) host, but it should not be too hard
-similar system. The host architecture should be x86_64 (amd64).
+to adapt to another similar system.
 
-We currently only build a single AppImage, for x86_64 architecture.
+We support building AppImages for both x86_64 (amd64) and aarch64 (ARM64) architectures.
-Help to adapt these scripts to build for (some flavor of) ARM would be welcome,
+**Important**: The host architecture must match the target architecture (i.e., build
-see [issue #5159](https://github.com/spesmilo/electrum/issues/5159).
+x86_64 AppImages on x86_64 hosts, and aarch64 AppImages on aarch64 hosts like Raspberry Pi).
 
 
 1. Install Docker
@@ -22,28 +24,44 @@ see [issue #5159](https://github.com/spesmilo/electrum/issues/5159).
 
 2. Build binary
 
+    **For x86_64 (amd64):**
     ```
     $ ./build.sh
     ```
-    If you want reproducibility, try instead e.g.:
+    Or for reproducibility:
     ```
     $ ELECBUILD_COMMIT=HEAD ./build.sh
     ```
 
+    **For aarch64 (ARM64):**
+    ```
+    $ ARCH=aarch64 ./build.sh
+    ```
+    Or for reproducibility:
+    ```
+    $ ARCH=aarch64 ELECBUILD_COMMIT=HEAD ./build.sh
+    ```
+
+    **Note**: By default, `ARCH` is set to `x86_64` if not specified.
+
 3. The generated binary is in `./dist`.
 
+    Output files are named with the architecture suffix:
+    - `pallectrum-<version>-x86_64.AppImage`
+    - `pallectrum-<version>-aarch64.AppImage`
+
 
 ## FAQ
 
 ### How can I see what is included in the AppImage?
-Execute the binary as follows: `./electrum*.AppImage --appimage-extract`
+Execute the binary as follows: `./pallectrum*.AppImage --appimage-extract`
 
 ### How to investigate diff between binaries if reproducibility fails?
 ```
 cd dist/
-./electrum-*-x86_64.AppImage1 --appimage-extract
+./pallectrum-*-x86_64.AppImage1 --appimage-extract
 mv squashfs-root/ squashfs-root1/
-./electrum-*-x86_64.AppImage2 --appimage-extract
+./pallectrum-*-x86_64.AppImage2 --appimage-extract
 mv squashfs-root/ squashfs-root2/
 $(cd squashfs-root1; find -type f -exec sha256sum '{}' \; > ./../sha256sum1)
 $(cd squashfs-root2; find -type f -exec sha256sum '{}' \; > ./../sha256sum2)
@@ -51,6 +69,8 @@ diff sha256sum1 sha256sum2 > d
 cat d
 ```
 
+(Replace `x86_64` with `aarch64` for ARM64 builds)
+
 For file metadata, e.g. timestamps:
 ```
 rsync -n -a -i --delete squashfs-root1/ squashfs-root2/