Flatten published Docker image (#6812)

This flattens the Docker image from 9 layers to 5 by using multi-stage
build that squashes layers into logical blocks. The first layer on top
of the base image adds system-wide packages and uv (which is not updated
so often - if it were, it may be wise to move it into the next or
separate layer; it weights roughly 50 MB) which should be preserved
between releases, while the next layer adds all Supervisor Python code
and dependencies.

This means that unless the base image or packages installed in the first
stage are changed (or in other words, only Supervisor code is changed),
only a single layer is pulled from the repository. Previously, it
generally resulted in pull of all the following 4 layers, as just a
change in the requirements invalidated the following layers. The fetched
payload size remains roughly the same.

Author: sairon

Dockerfile

@@ -1,14 +1,13 @@
 ARG BUILD_FROM=ghcr.io/home-assistant/base-python:3.14-alpine3.22-2026.04.0
-FROM ${BUILD_FROM}
+FROM ${BUILD_FROM} AS supervisor-base
 
 ENV \
     S6_SERVICES_GRACETIME=10000 \
     SUPERVISOR_API=http://localhost \
     CRYPTOGRAPHY_OPENSSL_NO_LEGACY=1 \
     UV_SYSTEM_PYTHON=true
 
-# Install base
-WORKDIR /usr/src
+# Install OS packages used both by build and final image
 RUN \
     set -x \
     && apk add --no-cache \
@@ -24,6 +23,14 @@ RUN \
     \
     && pip3 install uv==0.10.9
 
+#############################################
+# Install requirements and build Supervisor #
+#############################################
+
+FROM supervisor-base AS supervisor-build
+
+WORKDIR /usr/src
+
 # Install requirements
 RUN \
     --mount=type=bind,source=./requirements.txt,target=/usr/src/requirements.txt \
@@ -47,10 +54,18 @@ RUN \
     && uv pip install --no-cache -e ./supervisor \
     && python3 -m compileall ./supervisor/supervisor
 
-
-WORKDIR /
+# Copy the rest of rootfs files
 COPY rootfs /
 
+#########################
+# Final flattened image #
+#########################
+
+FROM supervisor-base
+
+# Copy everything from the build stage as a single layer
+COPY --from=supervisor-build / /
+
 LABEL \
     io.hass.type="supervisor" \
     org.opencontainers.image.title="Home Assistant Supervisor" \