I've updated the way I configure my backend image.
My Dockerfile now looks like this:
FROM plone/plone-backend:6.0.0b2
COPY src/site.content /app/src/site.content
COPY src/site.customviews /app/src/site.customviews
COPY src/site.plone /app/src/site.plone
COPY src/site.profile /app/src/site.profile
COPY src/plonetheme.fourteen /app/src/plonetheme.fourteen
COPY requirements.txt /app/requirements.txt
RUN apt-get update \
&& apt-get install -y --no-install-recommends gcc \
&& rm -rf /var/lib/apt/lists/*
RUN /app/bin/pip install -r /app/requirements.txt
I place a requirements.txt in the same folder as the Dockerfile and the requirements.txt now does all the "heavy lifting" as it should.
It looks like this:
-r https://dist.plone.org/release/6.0.0b2/constraints.txt
-e /app/src/site.content
-e /app/src/site.customviews
-e /app/src/site.plone
-e /app/src/site.profile
-e /app/src/plonetheme.fourteen
collective.exportimport==1.5
collective.easyform==4.1.0
Note the remote inclusion of the Plone 6 constraints.txt (very important). Also, my working assumption is that I need to apt-get gcc because my addons require compiling of stuff.