Small file that installes ad uses weasyprint instead of wkhtmltopdf on linuxserver bookstack containers..

install-weasyproxy.sh

#!/bin/bash
# SPDX-FileCopyrightText: 2022 Peter Magnusson <me@kmpm.se>
#
# SPDX-License-Identifier: CC0-1.0
#
# This file installs weasyprint and creates a script that
# tries to proxy calls from bookstack to what it believe is
# wkhtmltopdf.
#
# put this file in /custom-cont-init.d and restart the container
#
# add the following to /config/www/.env
# WKHTMLTOPDF=/usr/local/bin/weasyproxy
# ALLOW_UNTRUSTED_SERVER_FETCHING=true
#

# install weasyprint
apk add --update --no-cache weasyprint


# create the weasyproxy script
cat << EOF > /usr/local/bin/weasyproxy
#!/bin/bash
# this tries to work as a proxy between wkhtmltopdf and weasyprint
# for bookstack to use on for example the linuxserver alpine image

set -o errexit -o pipefail -o noclobber -o nounset
LOGFILE=/config/log/weasyproxy.log
echo "--- weasyproxy \$(date) ---" | tee -a \$LOGFILE
echo "inbound arguments: \$@" | tee -a \$LOGFILE

# -allow a command to fail with !’s side effect on errexit
# -use return value from \${PIPESTATUS[0]}, because ! hosed \$?
! getopt --test > /dev/null 
if [[ \${PIPESTATUS[0]} -ne 4 ]]; then
    echo 'I’m sorry, `getopt --test` failed in this environment.'
    exit 1
fi

LONGOPTS=lowquality,page-size:,outline,print-media-type
OPTIONS=ls:

! PARSED=\$(getopt --options=\$OPTIONS --longoptions=\$LONGOPTS --name "\$0" -- "\$@")
if [[ \${PIPESTATUS[0]} -ne 0 ]]; then
    # e.g. return value is 1
    #  then getopt has complained about wrong arguments to stdout
    exit 2
fi

eval set -- "\$PARSED"
PAGE_SIZE=Letter
OUTLINE=n
MEDIA=screen
ARGS="-p" # add -v for verbose and -d for debug output

while true; do
    case "\$1" in 
        -l|--lowquality)
            ARGS="\$ARGS --optimize-images"
            shift
            ;;
        -p|--page-size)
            PAGE_SIZE=\$2
            shift 2
            ;;
        --outline)
            OUTLINE=y
            shift
            ;;
        --print-media-type)
            MEDIA=print
            shift
            ;;
        --)
            shift
            break
            ;;
        *)
            echo "unsupported argument '\$1'" | tee -a \$LOGFILE
            exit 3
            ;;
    esac
done

ARGS="\$ARGS -s /config/weasyproxy/\${PAGE_SIZE}.css --media-type \${MEDIA}"

echo "PAGE_SIZE       = \${PAGE_SIZE}" | tee -a \$LOGFILE
echo "OUTLINE         = \${OUTLINE}" | tee -a \$LOGFILE
echo "MEDIA           = \${MEDIA}" | tee -a \$LOGFILE
echo "ARGS            = \${ARGS}" | tee -a \$LOGFILE


if [[ \$# -ne 2 ]]; then
    echo "\$0: input and output arguments is required" | tee -a \$LOGFILE
    exit 4
fi

weasyprint \${ARGS} \$1 \$2 >>\$LOGFILE 2>&1

EOF


# create css files for page sizes, if missing
if [ ! -d /config/weasyproxy ]; then
    mkdir /config/weasyproxy
fi

if [ ! -f /config/weasyproxy/A4.css ]; then
cat << EOF > /config/weasyproxy/A4.css 
@page {
  size: A4; 
}
EOF
fi

if [ ! -f /config/weasyproxy/Letter.css ]; then
cat << EOF > /config/weasyproxy/Letter.css 
@page {
  size: Letter;
}
EOF
fi

chmod +x /usr/local/bin/weasyproxy

Hey @kmpm thank you for this!
I found you on Linuxserver's issues (linuxserver/docker-bookstack#134) and it works wonderfully.
Only one note tho, I had to change
ARGS="\$ARGS --optimize-images"
from line 59 to
ARGS="\$ARGS --optimize-size images"

I suppose this changed overtime.
Many thanks of course, again!