MT#64132 Refresh docker image + update grml2usb to v0.20.14

Our latest two changes were:

  commit 3dfe3c7010
  MT#64132 Update to grml2usb version v0.20.13

  commit 0755c0e67b
  MT#64132 Update grml-live to version 0.56.0

But neither of those reached our relevant grml-build-trixie docker
image, as can be verified via:

  % docker run -it --rm docker.mgm.sipwise.com:5000/grml-build-trixie:latest
  Unable to find image 'docker.mgm.sipwise.com:5000/grml-build-trixie:latest' locally
  latest: Pulling from grml-build-trixie
  714eb16ffcfb: Pull complete
  a507fdebbccf: Pull complete
  756766956bb7: Pull complete
  87bae35a5cda: Pull complete
  65f90e2f6f13: Pull complete
  bbf06d44216e: Pull complete
  Digest: sha256:0e6f1a22888318316eb76986416f26dced9b5f16cd6dcb44b09cffe47b2d8945
  Status: Downloaded newer image for docker.mgm.sipwise.com:5000/grml-build-trixie:latest
  root@0c04a6dcc975:/code/grml-live# git log
  commit 28272472d6242608ddaa227499c6d878fddc1fa8 (grafted, HEAD, tag: v0.55.1)
  Author: Chris Hofstaedtler <ch@grml.org>
  Date:   Thu Dec 11 18:41:18 2025 +0100

      Update changelog for 0.55.1 release
  root@0c04a6dcc975:/code/grml-live#

The latest two commits modified files build_iso.sh +
grml_build/Dockerfile, but not the main t/Dockerfile one.

While we're detecting any `Dockerfile` changes as expected during review
time (see
https://jenkins.mgm.sipwise.com/job/deployment-iso-manage-docker/2568/console),
for yet unknown reasons the grml_build/Dockerfile modification was *not*
detected as such when submitting to master (see
https://jenkins.mgm.sipwise.com/job/deployment-iso-manage-docker/2571/console).

For now let's fix this by updating t/Dockerfile also, and while at it
update grml2usb to its latest upstream version v0.20.14, FTR:

  % git log --oneline --no-merges v0.20.13..v0.20.14
  4bbfeb9 (tag: v0.20.14, origin/master, origin/HEAD, master) Update changelog for 0.20.14 release
  c96f5b5 (origin/mika/grub-warns) On failing grub-install, point users to according GRUB packages
  3f951a8 (origin/mika/grub-arm64) Drop depends on arm64 specific grub packages
  d7baed8 (origin/mika/grub) Adjust Depends to improve grub-pc-bin situation

Change-Id: I14a243993bcccad595fa8832d178b5b8af594540

build_iso.sh

@@ -16,7 +16,7 @@ TEMPLATES="templates"
 GRML_URL="${GRML_URL:-https://deb.sipwise.com/deployment-iso/grml/}"
 GRML_HASH_URL="${GRML_HASH_URL:-https://deb.sipwise.com/deployment-iso/grml/}"
 SIPWISE_ISO="sip_provider_${MR}_${DATE}.iso"
-GRML2USB_VERSION='v0.20.13'
+GRML2USB_VERSION='v0.20.14'
 
 usage () {
   echo "Usage: $0 compat <grml.iso> <mr version> <Debian dist>"

t/Dockerfile

@@ -5,7 +5,7 @@ FROM docker.mgm.sipwise.com/sipwise-trixie:latest
 # is updated with the current date. It will force refresh of all
 # of the base images and things like `apt-get update` won't be using
 # old cached versions when the Dockerfile is built.
-ENV REFRESHED_AT=2026-01-30
+ENV REFRESHED_AT=2026-06-09
 
 RUN apt-get update && apt-get install --assume-yes git make gcc dpkg-dev