feat: optimize binary size, crun, zstd compression, online/offline build variants

[nova8-technologies]

Description

Reduces KubeSolo binary size by ~160–180 MB for the default (online) build variant through three changes:

Phase 1: Replace runc with crun

Replace runc (~10 MB) with crun (~1 MB) as the OCI runtime. crun is a lightweight C-based runtime that is fully OCI-compliant and used as the default in Podman/Buildah. It supports cgroupv2 natively.

Note: crun does not publish pre-built 32-bit ARM (armhf) binaries. The download-deps.sh script will fail early with a clear error for GOARCH=arm.

Phase 2: zstd-compress embedded binaries

Compress containerd-shim-runc-v2, crun, and CNI plugin binaries with zstd -19 before go:embed. Binaries are decompressed at extraction time using github.com/klauspost/compress/zstd (already an indirect dependency, promoted to direct).

Phase 3: Online/offline build variants via build tags

Split into two build modes:

Variant	Build Tag	Embedded Images	Runtime Behavior
Online (default)	none	CoreDNS + pause only	Portainer Agent + local-path-provisioner pulled via client.Pull()
Offline	-tags offline	All images	Air-gapped deployments, no network needed

The importImage() function now falls back to client.Pull() when an embedded image file is empty or missing, making the online variant seamless.

Build Commands

# Online build (default, ~160-180 MB smaller)
make build GOOS=linux GOARCH=amd64

# Offline build (all images embedded, air-gapped)
make build-offline GOOS=linux GOARCH=amd64

# Download deps for offline
make deps-offline

Files Changed

File	Change
Makefile	Add build-offline and deps-offline targets
build/download-deps.sh	crun download, zstd compression, --offline flag
cmd/kubesolo/main.go	RuncBinaryFile → CrunBinaryFile
go.mod	Promote klauspost/compress to direct dependency
internal/core/embedded/embedded.go	.zst suffixes, remove always-embedded optional images
internal/core/embedded/embedded_riscv64.go	Same .zst changes for riscv64
internal/core/embedded/embedded_images_offline.go	New embed optional images with //go:build offline
internal/core/embedded/embedded_images_offline_riscv64.go	New offline riscv64 (no portainer-agent)
internal/core/embedded/embedded_images_online.go	New empty vars for online build
internal/core/embedded/load.go	runcBinary → crunBinary
internal/runtime/filesystem/binary.go	zstd decompression in ExtractBinary()
pkg/runtime/containerd/config.go	Runtime name runc → crun, cgroupv2 detection
pkg/runtime/containerd/image.go	Fallback to client.Pull() for missing images
pkg/runtime/containerd/service.go	runcBinaryFile → crunBinaryFile
types/types.go	RuncBinaryFile → CrunBinaryFile

Testing

• Online build: verified CoreDNS/pause load from embedded, portainer-agent/local-path-provisioner pulled at runtime
• Offline build: verified all images load from embedded tar.gz files
• Verified zstd decompression produces identical binaries to originals
• Tested on amd64 and arm64

[stevensbkang]

Would you please consider the comment?

[stevensbkang]

We encountered the exact same limitation for containerd artefacts and ended up building our own https://github.com/portainer/kubesolo/blob/develop/build/containerd.Dockerfile.

I have done some testing and the following seems to do the job:

# crun build for arm/v7 using Debian Linux (glibc)
#
# Strategy: Build natively inside an arm/v7 Debian container via QEMU.
# This produces a glibc-linked binary that matches the current installer
# expectations for linux-arm artifacts.
#
# Build with: docker build --platform linux/arm/v7 -f ./build/crun.Dockerfile -t crun-arm32-builder .
# Extract with:
#   docker create --name crun-extract crun-arm32-builder noop
#   docker cp crun-extract:/crun ./internal/core/embedded/bin/crun
#   docker rm crun-extract

FROM --platform=linux/arm/v7 debian:12-slim AS builder

ARG CRUN_VERSION=1.26

# Install build dependencies
RUN apt-get update && \
    apt-get install -y --no-install-recommends \
        autoconf \
        automake \
        build-essential \
        ca-certificates \
        git \
        go-md2man \
        gperf \
        libcap-dev \
        libseccomp-dev \
        libsystemd-dev \
        libtool \
        linux-libc-dev \
        pkg-config \
        python3 \
        && rm -rf /var/lib/apt/lists/*

# Clone crun repository
RUN git clone --recursive https://github.com/containers/crun.git /src/crun
WORKDIR /src/crun

# Checkout specific version
RUN git checkout ${CRUN_VERSION} && \
    git submodule update --init --recursive

# Generate configure script
RUN ./autogen.sh

# Configure for glibc build with systemd support to match upstream features
RUN ./configure \
    --enable-systemd \
    --enable-embedded-yajl \
    --with-cap \
    --with-seccomp

# Build
RUN make -j"$(nproc)"

# Package output
RUN mkdir -p /output/bin && \
    cp crun /output/bin/ && \
    strip /output/bin/crun

# Final stage: copy the built binary to a clean image
FROM scratch
COPY --from=0 /output/bin/crun /

Testing on a arm/v7 RPi:

root@pi2b:~# dpkg --print-architecture
armhf
root@pi2b:~# uname -m
armv7l
root@pi2b:/tmp# ./crun --version
crun version 1.26
commit: 3241e671f92c33b0c003cd7de319e4f32add6231
rundir: /run/crun
spec: 1.0.0
+SYSTEMD +SELINUX +APPARMOR +CAP +SECCOMP +EBPF +YAJL

Would you be able to add this to the PR, please? I will take care of the release to produce offline vs online in a separate PR 😄