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

########## 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