Measure Docker build context size across legacy Docker, BuildKit, and Podman (with/without .dockerignore)

measure-docker-context.sh

#!/usr/bin/env bash
# Measure Docker build context size with and without .dockerignore
# Covers: legacy Docker builder, BuildKit (via buildx), and Podman
#
# Run from the repo root where .dockerignore lives.
# The script temporarily moves .dockerignore aside to measure "without",
# then restores it to measure "with".

set -uo pipefail

DOCKERIGNORE=".dockerignore"
BACKUP=".dockerignore.bak"
MEASURE_DOCKERFILE=$(mktemp /tmp/ctx-measure.XXXXXX.Dockerfile)

trap 'rm -f "$MEASURE_DOCKERFILE"; [ -f "$BACKUP" ] && mv "$BACKUP" "$DOCKERIGNORE"' EXIT

cat > "$MEASURE_DOCKERFILE" <<'DEOF'
FROM busybox
COPY . /ctx
RUN du -sh /ctx
DEOF

without_dockerignore() {
  [ -f "$DOCKERIGNORE" ] && mv "$DOCKERIGNORE" "$BACKUP"
}

with_dockerignore() {
  [ -f "$BACKUP" ] && mv "$BACKUP" "$DOCKERIGNORE"
}

# Extract "1.5G /ctx" from BuildKit/Podman output where du is prefixed
# by build step metadata (e.g. "#7 0.315 1.5G /ctx" or bare "1.5G /ctx")
parse_du() {
  grep -oE "[0-9][0-9.]*[KMGT][[:space:]]*/ctx" | head -1 || echo "(could not parse du output)"
}

# ---------- Legacy Docker builder ----------
echo "########## Legacy Docker builder (DOCKER_BUILDKIT=0) ##########"
echo ""

without_dockerignore
echo "--- Without .dockerignore ---"
DOCKER_BUILDKIT=0 docker build --no-cache -f - . <<< 'FROM scratch' 2>&1 \
  | { grep "Sending build context" || echo "(no context size reported)"; }

with_dockerignore
echo "--- With .dockerignore ---"
DOCKER_BUILDKIT=0 docker build --no-cache -f - . <<< 'FROM scratch' 2>&1 \
  | { grep "Sending build context" || echo "(no context size reported)"; }

echo ""

# ---------- BuildKit (via docker buildx) ----------
echo "########## BuildKit (docker buildx build --progress=plain) ##########"
echo ""
echo "Note: BuildKit does not print context size on its own."
echo "Using COPY . /ctx + du -sh to measure what arrives."
echo ""

without_dockerignore
echo "--- Without .dockerignore ---"
docker buildx build --progress=plain --no-cache -f "$MEASURE_DOCKERFILE" -t ctx-measure-bk . 2>&1 \
  | parse_du

with_dockerignore
echo "--- With .dockerignore ---"
docker buildx build --progress=plain --no-cache -f "$MEASURE_DOCKERFILE" -t ctx-measure-bk . 2>&1 \
  | parse_du

docker rmi ctx-measure-bk >/dev/null 2>&1 || true

echo ""

# ---------- Podman ----------
echo "########## Podman ##########"
echo ""
echo "Note: Podman does not report context size. Using COPY . /ctx + du -sh."
echo ""

without_dockerignore
echo "--- Without .dockerignore ---"
podman build --no-cache -f "$MEASURE_DOCKERFILE" -t ctx-measure-pm . 2>&1 \
  | parse_du

with_dockerignore
echo "--- With .dockerignore ---"
podman build --no-cache -f "$MEASURE_DOCKERFILE" -t ctx-measure-pm . 2>&1 \
  | parse_du

podman rmi ctx-measure-pm >/dev/null 2>&1 || true

Results on kserve/kserve repo

Context sizes

Builder	Without .dockerignore	With .dockerignore
Legacy Docker	1.572 GB	11.85 MB
BuildKit (buildx)	1.5 GB	11.9 MB
Podman	1.5 GB	11.9 MB

Where the 1.5 GB comes from

The Go Dockerfiles only need ~12 MB of source, but without an allowlist .dockerignore the entire repo gets sent as context:

Directory	Size	Needed by Go builds?
bin/	452 MB	No - compiled binaries
.git/	373 MB	No - VCS metadata
python/	282 MB	No - Python images use python/ as their own context
venv/	190 MB	No - local virtualenv
install/	117 MB	No - install manifests
hack/	32 MB	No - dev scripts
docs/	30 MB	No - documentation
test/	20 MB	No - test fixtures
pkg/	6.7 MB	Yes
tools/	5.0 MB	Yes
cmd/	124 KB	Yes
qpext/	80 KB	Yes
go.mod + go.sum	84 KB	Yes
LICENSE	12 KB	Yes

The allowlist .dockerignore includes only the last 6 rows, cutting context by 99.2%.

Note: BuildKit does not exclude .git/ automatically despite common belief - all three builders send the same ~1.5 GB without a .dockerignore.

########## Legacy Docker builder (DOCKER_BUILDKIT=0) ##########

--- Without .dockerignore ---
Sending build context to Docker daemon  1.572GB
--- With .dockerignore ---
Sending build context to Docker daemon  11.85MB

########## BuildKit (docker buildx build --progress=plain) ##########

Note: BuildKit does not print context size. Using COPY . /ctx + du -sh to measure what arrives.

--- Without .dockerignore ---
1.5G    /ctx
--- With .dockerignore ---
11.9M   /ctx

########## Podman ##########

Note: Podman does not report context size. Using COPY . /ctx + du -sh.

--- Without .dockerignore ---
1.5G    /ctx
--- With .dockerignore ---
11.9M   /ctx