dockerized landing build doesn't do anything

[computermouth]

Here: https://github.com/digitalocean/sample-monorepo/blob/main/.do/app.yaml#L16
And Here: https://github.com/digitalocean/sample-monorepo/blob/main/landing/Dockerfile

The dockerfile adds the static assets to /app, and seemingly the contents of /app get copied out and added to the group of static assets. But if I add anything to my Dockerfile, for example

FROM alpine
ADD . /app/
WORKDIR /app
RUN echo "howdy" >> index.html

The contents of the file don't change. If I do the docker image build on my machine, I see the intended output

$ docker build -t some .
Sending build context to Docker daemon  3.072kB
Step 1/4 : FROM alpine
 ---> b2aa39c304c2
Step 2/4 : ADD . /app/
 ---> b973e426495f
Step 3/4 : WORKDIR /app
 ---> Running in d9c5403f821d
Removing intermediate container d9c5403f821d
 ---> 566ea7d8d0e4
Step 4/4 : RUN echo howdy >> index.html
 ---> Running in bf1f6ceb0464
Removing intermediate container bf1f6ceb0464
 ---> 09d3bb4b5cfb
Successfully built 09d3bb4b5cfb
Successfully tagged some:latest

$ docker run -it --rm some cat /app/index.html
Welcome!
howdy

But at my static site, after a build

Welcome!

So what gives? This pattern of adding stuff into a Docker container, theoretically manipulating it, and then it gets copied out of the image afterward is pretty unique/odd, even if it did work, but it seems like it doesn't.

[computermouth]

The build is succeeding, landing/index.html is getting added, Custom Build Command Buildpack is seemingly running, it says No build command defined, and the Buildpack Docs link there is broken: https://docs.digitalocean.com/products/app-platform/concepts/command/