Yes - same as with the original script, upgrades would require more manual steps than just updating the version in the Compose file. This is how it's typically done.

There are ways to automate this. Docker Hub used to have a feature for automatic rebuilds when base images had changed, but AFAIK this feature was removed some years ago. Now it's a matter of setting up a CI with periodically (nightly or weekly) scheduled pipelines, but it's not a trivial matter.

Semi-automation can be achieved by using build-time arguments. I'm at my computer now, so here's a revised process:

First, a bunch of manual commands that would allow us to write a patch. I'll use those crude sed statements - just because they work today, but YMMV.

docker run -it --rm --user root tootsuite/mastodon bash
cp -r /opt/mastodon /opt/mastodon.vanilla
sed -i 's/500/1000/g' /opt/mastodon/app/javascript/mastodon/features/compose/components/compose_form.js
sed -i 's/500/1000/g' /opt/mastodon/app/validators/status_length_validator.rb
diff -urN /opt/mastodon.vanilla /opt/mastodon

This will produce a nice patch like this that you can copy. Create an empty directory and save as change-limits.patch in there:

diff -urN /opt/mastodon.vanilla/app/javascript/mastodon/features/compose/components/compose_form.js /opt/mastodon/app/javascript/mastodon/features/compose/components/compose_form.js
***
/opt/mastodon.vanilla/app/javascript/mastodon/features/compose/components/compose_form.js   2023-07-07 17:50:26.046682458 +0000
+++ /opt/mastodon/app/javascript/mastodon/features/compose/components/compose_form.js   2023-07-07 17:50:49.626674917 +0000
@@ -90,7 +90,7 @@
     const fulltext = this.getFulltextForCharacterCounting();
     const isOnlyWhitespace = fulltext.length !== 0 && fulltext.trim().length === 0;

-    return !(isSubmitting || isUploading || isChangingUpload || length(fulltext) > 500 || (isOnlyWhitespace && !anyMedia));
+    return !(isSubmitting || isUploading || isChangingUpload || length(fulltext) > 1000 || (isOnlyWhitespace && !anyMedia));
   };

   handleSubmit = (e) => {
@@ -280,7 +280,7 @@
           </div>

           <div className='character-counter__wrapper'>
-            <CharacterCounter max={500} text={this.getFulltextForCharacterCounting()} />
+            <CharacterCounter max={1000} text={this.getFulltextForCharacterCounting()} />
           </div>
         </div>

diff -urN /opt/mastodon.vanilla/app/validators/status_length_validator.rb /opt/mastodon/app/validators/status_length_validator.rb
***
/opt/mastodon.vanilla/app/validators/status_length_validator.rb     2023-07-07 17:50:26.106682438 +0000
+++ /opt/mastodon/app/validators/status_length_validator.rb     2023-07-07 17:51:00.796671166 +0000
@@ -1,7 +1,7 @@
 # frozen_string_literal: true

 class StatusLengthValidator < ActiveModel::Validator
-  MAX_CHARS = 500
+  MAX_CHARS = 1000
   URL_PLACEHOLDER_CHARS = 23
   URL_PLACEHOLDER = 'x' * 23

Now, put the Dockerfile next to the patch:

ARG MASTODON_VERSION=latest
FROM tootsuite/mastodon:${MASTODON_VERSION}

USER root
RUN apt-get update && apt-get install -y --no-install-recommends patch
COPY change-limits.patch ./
RUN patch -up3 < change-limits.patch

USER mastodon
RUN OTP_SECRET=precompile_placeholder SECRET_KEY_BASE=precompile_placeholder rails assets:precompile

And a shell script to semi-automate the upgrades. Note it requires curl and jq to be available to parse JSON.

#!/bin/sh

set -e

MASTODON_VERSION="$(curl -s "https://hub.docker.com/v2/namespaces/tootsuite/repositories/mastodon/tags?page_size=100" | jq -r '.results[]["name"]' | sort -rV | head -n 1)"
echo "Latest version: ${MASTODON_VERSION}"
docker pull "tootsuite/mastodon:${MASTODON_VERSION}"
docker build --build-arg "MASTODON_VERSION=${MASTODON_VERSION}" -t "my-mastodon:${MASTODON_VERSION}" .

And finally, create a file called .dockerignore that contains only one line that would say build.sh. That's just minor cosmetic touch saying that our build.sh is not meant to be a part of the build context. If everything is correct, there should be now 4 files in the directory: .dockerignore, build.sh, change-limits.patch and Dockerfile.

Now when you run build.sh it will automatically find the latest version and build you a custom image tagged as e.g. my-mastodon:v4.1.3, which you can use in your Compose file. For a distributed system like Docker Swarm, Nomad or Kubernetes you'll want to tweak this script a little to use some registry (your-registry.example.org/your/mastodon:v4.1.3) and possibly even apply changes further (e.g. docker service update --image ...).

Mutable tags like latest can become confusing or even problematic (e.g. if you'll want to downgrade it's best to have previous version image readily available for some time - even if the build process is reproducible), so I would really recommend to use explicit version number tags.

Hope this helps.