Build libvips with a GitHub Actions workflow (#31)

The version of libvips can be defined in the `LIBVIPS_VERSION` file in the root of the repository.

Every time a new tag starting with 'v*', such as `v8.8.1` is being pushed, the "build-release" workflow will be triggered and do the following things:

* Create a pre-release for the tag on GitHub
* Build libvips with the version provided in `LIBVIPS_VERSION` for every platform
* Upload the release artifacts to the GitHub release which was created before

Author: joschi

.github/workflows/build-release.yml

@@ -0,0 +1,63 @@
+name: Build Release
+on:
+  push:
+    tags:
+    - v*
+jobs:
+  create-release:
+    runs-on: 'ubuntu-latest'
+    steps:
+    - name: Create Release
+      id: create-release
+      uses: actions/create-release@v1
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      with:
+        tag_name: ${{ github.ref }}
+        release_name: Release ${{ github.ref }}
+        draft: false
+        prerelease: true
+    - shell: bash
+      run: |
+        echo '${{ steps.create-release.outputs.upload_url }}' > upload-url.txt
+    - name: Save GitHub release upload URL for next job
+      uses: actions/upload-artifact@v1
+      with:
+        name: upload-url
+        path: upload-url.txt
+  build:
+    runs-on: 'ubuntu-latest'
+    needs:
+      - 'create-release'
+    strategy:
+      matrix:
+        platform:
+          - 'linux-x64'
+          - 'linuxmusl-x64'
+          - 'linux-armv6'
+          - 'linux-armv7'
+          - 'linux-arm64v8'
+          - 'win32-x64'
+    steps:
+    - uses: actions/checkout@v1
+    - name: Fetch GitHub Release upload URL
+      uses: actions/download-artifact@v1
+      with:
+        name: upload-url
+    - name: Set up environment variables
+      shell: bash
+      run: |
+        echo "::set-env name=upload_url::$(cat upload-url/upload-url.txt)"
+        echo "::set-env name=asset_file_name::libvips-$(cat LIBVIPS_VERSION)-${{ matrix.platform }}.tar.gz"
+    - name: Build ${{ matrix.platform }}
+      run: ./build.sh $(cat LIBVIPS_VERSION) ${{ matrix.platform }}
+    - name: Upload Release Asset
+      id: upload-release-asset 
+      uses: actions/[email protected]
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      with:
+        upload_url: ${{ env.upload_url }}
+        asset_path: ${{ env.asset_file_name }}
+        asset_name: ${{ env.asset_file_name }}
+        asset_content_type: application/gzip

LIBVIPS_VERSION

@@ -0,0 +1 @@
+8.8.1