Docker Build Slow and Not Using Cache: 5 Fixes

# BAD — copies entire source before installing dependencies
FROM node:20-alpine
WORKDIR /app
COPY . .                          # Invalidated by ANY file change
RUN npm install                   # Reinstalls all packages every time

# GOOD — install dependencies first, copy source last
FROM node:20-alpine
WORKDIR /app
 
# These layers only rebuild when package.json changes
COPY package.json package-lock.json ./
RUN npm ci --only=production
 
# This layer rebuilds on source changes, but npm install stays cached
COPY . .

FROM python:3.12-slim
WORKDIR /app
 
# Copy requirements first — cached until requirements.txt changes
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt
 
# Copy source last
COPY . .

FROM golang:1.22-alpine
WORKDIR /app
 
COPY go.mod go.sum ./
RUN go mod download          # Cached until go.mod/go.sum changes
 
COPY . .
RUN go build -o /app/server ./cmd/server

# Build with DOCKER_BUILDKIT=1 and check what's being sent
DOCKER_BUILDKIT=1 docker build . --progress=plain 2>&1 | head -20
# Look for "transferring context" — the size should be small (not hundreds of MB)

# Enable BuildKit
export DOCKER_BUILDKIT=1
 
# Or use docker buildx (always uses BuildKit)
docker buildx build .

# syntax=docker/dockerfile:1
FROM python:3.12-slim
 
WORKDIR /app
COPY requirements.txt .
 
# --mount=type=cache persists pip cache between builds
# Even on CI with a fresh container, cache is preserved on the host
RUN --mount=type=cache,target=/root/.cache/pip \
    pip install -r requirements.txt
 
COPY . .

RUN --mount=type=cache,target=/var/cache/apt \
    apt-get update && apt-get install -y curl wget

RUN --mount=type=cache,target=/root/.npm \
    npm ci --prefer-offline

# syntax=docker/dockerfile:1
 
# Stage 1: Build
FROM golang:1.22-alpine AS builder
WORKDIR /build
 
COPY go.mod go.sum ./
RUN --mount=type=cache,target=/go/pkg/mod \
    go mod download
 
COPY . .
RUN --mount=type=cache,target=/root/.cache/go-build \
    CGO_ENABLED=0 go build -ldflags="-w -s" -o server ./cmd/server
 
# Stage 2: Final image — only the binary
FROM gcr.io/distroless/static-debian12
COPY --from=builder /build/server /server
ENTRYPOINT ["/server"]

name: Build and Push
 
on:
  push:
    branches: [main]
 
jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
 
      - name: Set up Docker Buildx
        uses: docker/setup-buildx-action@v3
 
      - name: Login to ECR
        uses: aws-actions/amazon-ecr-login@v2
 
      - name: Build and push
        uses: docker/build-push-action@v5
        with:
          context: .
          push: true
          tags: 123456789.dkr.ecr.ap-south-1.amazonaws.com/myapp:${{ github.sha }}
          # Cache from/to GitHub Actions cache
          cache-from: type=gha
          cache-to: type=gha,mode=max

cache-from: type=registry,ref=123456789.dkr.ecr.ap-south-1.amazonaws.com/myapp:cache
cache-to: type=registry,ref=123456789.dkr.ecr.ap-south-1.amazonaws.com/myapp:cache,mode=max

# Check if cache is being used
docker build . --progress=plain 2>&1 | grep -E "CACHED|RUN"
# Lines starting with "CACHED" are cache hits
# Lines starting with "RUN" are cache misses
 
# Check context size being sent to Docker daemon
docker build . 2>&1 | grep "Sending build context"
# Should be KB, not MB
 
# Check layer history
docker history myapp:latest --no-trunc
 
# Force rebuild without cache (to establish baseline)
docker build --no-cache .