do not use untrusted docker image

Jenkinsfile

@@ -21,20 +21,18 @@ pipeline {
         {
             agent {
                 docker {
-                    image 'dpokidov/imagemagick'
+                    image 'alpine:3.21'
                     reuseNode true
                 }
             }
             steps {
                 sh '''
                     set -eu
-
+                    apk add --no-cache imagemagick
                     found_edition=0
-
                     for edition_dir in editions/*; do
                         [ -d "$edition_dir" ] || continue
                         found_edition=1
-
                         set -- "$edition_dir"/splash-source.*
                         if [ ! -e "$1" ]; then
                             echo "ERROR: missing splash-source.* in $edition_dir" >&2
@@ -44,12 +42,9 @@ pipeline {
                             echo "ERROR: multiple splash-source.* files in $edition_dir" >&2
                             exit 1
                         fi
-
                         src="$1"
                         out="$edition_dir/splash.jpg"
-
                         echo "Converting $src -> $out"
-
                         magick "$src" \
                             -resize 640x480! \
                             -background black \