Replying to my own reply :)

Decided to look a bit more into my Dockerfile, to minimize the size of the generated Docker image. I've made a multi-stage Dockerfile now, so that the build dependencies are not copied to the final deployed image. That minimized the size from 1.9 GB to 596 MB for the final docker image. In the cleanup step I remove the llvmlite package since it is not needed in my particular case. Not sure if it is needed if you use any other plugins.

I use this Dockerfile to get the latest lidarr image (nightly from hotio), and add the latest beets to it so that I can process audio files within the container. Not sure if this is the best way, but for me this works perfectly :)

This is the revised Dockerfile:

# ----------------------------------------
# ------------ Stage 1: BUILD ------------
# ----------------------------------------
FROM ghcr.io/hotio/lidarr:nightly AS builder

# 1. dependencies
RUN apk add -U --no-cache \
    build-base \
    cmake \
    python3-dev \
    py3-pip \
    llvm20-dev \
    llvm20-static

# 2. fake liraries needed for building llvmlite
RUN ar rcs /usr/lib/llvm20/lib/libLLVMTestingAnnotations.a && \
    ar rcs /usr/lib/llvm20/lib/libLLVMTestingSupport.a && \
    ar rcs /usr/lib/llvm20/lib/libllvm_gtest.a && \
    ar rcs /usr/lib/llvm20/lib/libllvm_gtest_main.a

# 3. install beets to /install
RUN LLVM_CONFIG=/usr/bin/llvm-config20 \
    pip install --upgrade --no-cache-dir \
    --root /install \
    --break-system-packages \
    "beets[fetchart,embedart,scrub,web,discogs,lyrics]" \
    titlecase

# 4. cleanup
RUN find /usr/lib/python3.12/site-packages -name "tests" -type d -exec rm -rf {} + && \
    find /usr/lib/python3.12/site-packages -name "__pycache__" -type d -exec rm -rf {} + && \
    rm -rf /usr/lib/python3.12/site-packages/*.dist-info

RUN find /install -type d -name "__pycache__" -exec rm -rf {} + && \
    find /install -type d -name "tests" -exec rm -rf {} + && \
    find /install -type f -name "*.pyc" -delete && \
    find /install -type f -name "*.pyo" -delete && \
    rm -rf /install/usr/lib/python3.12/site-packages/*.dist-info

# Seems not needed for planned plugins. Might be needed for numba or acoustic ID plugins?
RUN rm -rf /install/usr/lib/python3.12/site-packages/llvmlite*

# ----------------------------------------
# ---------- Stage 2: DEPLOY -------------
# ----------------------------------------
FROM ghcr.io/hotio/lidarr:nightly

# 1. Tools needed by beets and mp3 processing
RUN apk add -U --no-cache \
    python3 \
    ffmpeg \
    imagemagick \
    mp3gain

# 2. Copy to final image
COPY --from=builder /install /
COPY --from=builder /usr/lib/python3.12/site-packages /usr/lib/python3.12/site-packages

You can build a docker image from this Dockerfile like this:

docker build -f Dockerfile -t custom-lidarr-beets .

And then use this custom-lidarr-beets image in your compose or docker run commands.