Reproducible builds with GitHub Actions

Table of contents

SOURCE_DATE_EPOCH is a standardized environment variable for instructing build tools to produce a reproducible output. Setting the environment variable for a build makes the timestamps in the image index, config, and file metadata reflect the specified Unix time.

To set the environment variable in GitHub Actions, use the built-in env property on the build step.

Unix epoch timestamps

The following example sets the SOURCE_DATE_EPOCH variable to 0, Unix epoch.

name: ci

on:
  push:

jobs:
  docker:
    runs-on: ubuntu-latest
    steps:
      - name: Set up Docker Buildx
        uses: docker/setup-buildx-action@v3
      
      - name: Build
        uses: docker/build-push-action@v6
        with:
          tags: user/app:latest
        env:
          SOURCE_DATE_EPOCH: 0

name: ci

on:
  push:

jobs:
  docker:
    runs-on: ubuntu-latest
    steps:
      - name: Checkout
        uses: actions/checkout@v4
      
      - name: Set up Docker Buildx
        uses: docker/setup-buildx-action@v3
      
      - name: Build
        uses: docker/bake-action@v5
        env:
          SOURCE_DATE_EPOCH: 0

Git commit timestamps

The following example sets SOURCE_DATE_EPOCH to the Git commit timestamp.

name: ci

on:
  push:

jobs:
  docker:
    runs-on: ubuntu-latest
    steps:
      - name: Set up Docker Buildx
        uses: docker/setup-buildx-action@v3
      
      - name: Get Git commit timestamps
        run: echo "TIMESTAMP=$(git log -1 --pretty=%ct)" >> $GITHUB_ENV
      
      - name: Build
        uses: docker/build-push-action@v6
        with:
          tags: user/app:latest
        env:
          SOURCE_DATE_EPOCH: ${{ env.TIMESTAMP }}

name: ci

on:
  push:

jobs:
  docker:
    runs-on: ubuntu-latest
    steps:
      - name: Checkout
        uses: actions/checkout@v4
      
      - name: Set up Docker Buildx
        uses: docker/setup-buildx-action@v3
      
      - name: Get Git commit timestamps
        run: echo "TIMESTAMP=$(git log -1 --pretty=%ct)" >> $GITHUB_ENV
      
      - name: Build
        uses: docker/bake-action@v5
        env:
          SOURCE_DATE_EPOCH: ${{ env.TIMESTAMP }}

Additional information

For more information about the SOURCE_DATE_EPOCH support in BuildKit, see BuildKit documentation.