Export ios app icons by ffmpeg scale command. It doesn't add alpha channel to png format image.

export_ios_icons.sh

# Export ios app icons by ffmpeg scale command
# usage: sh export_ios_icons.sh {path_to_your_img}
# example: sh export_ios_icons.sh ./app_icon.png

# sizes of images
# you can get other size images by editing thisarray
size=(20 40 60 29 58 87 80 120 180 76 152 167 1024)
for i in "${size[@]}"
do
   : 
    ffmpeg -i $1 -vf scale=$i:$i output_$ix$i.png
done