Build and push images with Github Actions to DockerHub (#68)

* Build and push images with Github Actions to DockerHub
* Remove Travis CI configuration

References:
* https://docs.github.com/en/actions/guides/publishing-docker-images
* https://github.com/docker/build-push-action/blob/master/docs/advanced/tags-labels.md
* https://github.com/docker/metadata-action

Note: docker / buildx does not support '+' in the image tag name.
> invalid tag "martialblog/limesurvey:0.0.0+test5-apache": invalid
reference format

Use underscores instead, like the images already have on Dockerhub.

Co-authored-by: Markus Opolka <markus.opolka@iis.fraunhofer.de>

.github/workflows/lint-dockerfiles.yaml

@@ -0,0 +1,24 @@
+---
+name: Lint Dockerfile
+
+on: [push, pull_request]
+
+jobs:
+  lint:
+    name: Lint Dockerfile with hadolint
+    strategy:
+      matrix:
+        dockerfile:
+          - 3.0/apache/Dockerfile
+          - 3.0/fpm-alpine/Dockerfile
+          - 3.0/fpm/Dockerfile
+          - 5.0/apache/Dockerfile
+          - 5.0/fpm-alpine/Dockerfile
+          - 5.0/fpm/Dockerfile
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: hadolint/hadolint-action@v1.5.0
+        with:
+          dockerfile: ${{ matrix.dockerfile }}
+          ignore: DL4006 DL3008 DL3018