Add GitHub Action for Flux CLI

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>

action/Dockerfile

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+FROM stefanprodan/alpine-base:latest
+
+COPY entrypoint.sh /entrypoint.sh
+RUN chmod +x /entrypoint.sh
+
+ENTRYPOINT ["/entrypoint.sh"]

action/README.md

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+# Flux GitHub Action
+
+Example workflow:
+
+```yaml
+name: e2e
+
+on:
+  push:
+    branches:
+      - '*'
+
+jobs:
+  kubernetes:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Setup Flux CLI
+        uses: fluxcd/flux2/action@main
+      - name: Setup Kubernetes Kind
+        uses: engineerd/setup-kind@v0.5.0
+      - name: Install Flux in Kubernetes Kind
+        run: flux install
+```

action/action.yml

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+name: 'kustomize'
+description: 'A GitHub Action for running Flux commands'
+author: 'Flux project'
+branding:
+  icon: 'command'
+  color: 'blue'
+inputs:
+  version:
+    description: 'strict semver'
+    required: false
+runs:
+  using: 'docker'
+  image: 'Dockerfile'
+  args:
+    - ${{ inputs.version }}

action/entrypoint.sh

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+#!/bin/bash
+
+#  Copyright 2020 The Flux authors
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+
+set -e
+
+VERSION=$1
+
+if [ -z $VERSION ]; then
+  # Find latest release if no version is specified
+  VERSION=$(curl https://api.github.com/repos/fluxcd/flux2/releases/latest -sL | grep tag_name | sed -E 's/.*"([^"]+)".*/\1/' | cut -c 2-)
+fi
+
+# Download linux binary
+BIN_URL="https://github.com/fluxcd/flux2/releases/download/v${VERSION}/flux_${VERSION}_linux_amd64.tar.gz"
+curl -sL $BIN_URL | tar xz
+
+# Copy binary to GitHub runner
+mkdir -p $GITHUB_WORKSPACE/bin
+cp ./flux $GITHUB_WORKSPACE/bin
+chmod +x $GITHUB_WORKSPACE/bin/flux
+
+# Print version
+$GITHUB_WORKSPACE/bin/flux -v
+
+# Add binary to GitHub runner path
+echo "$GITHUB_WORKSPACE/bin" >> $GITHUB_PATH
+echo "$RUNNER_WORKSPACE/$(basename $GITHUB_REPOSITORY)/bin" >> $GITHUB_PATH