Add docker image test

.docker/release/test/build.sh

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+#!/bin/sh
+set -eu
+docker build -t snappymail/snappymail:test -f .docker/release/Dockerfile .

.docker/release/test/config.yaml

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+# See: https://github.com/GoogleContainerTools/container-structure-test
+# See: https://github.com/GoogleContainerTools/container-structure-test/issues/78
+schemaVersion: 2.0.0
+commandTests:
+  - name: Server starts
+    command: /bin/sh
+    args:
+      - -c
+      - |
+          /entrypoint.sh > /tmp/out 2>&1 &
+          sleep 5
+          set -eux
+          cat /tmp/out
+          pidof supervisord
+          pidof nginx
+          pidof php-fpm
+          wget -S -T 3 -O /dev/null http://localhost:8888
+          kill `pidof supervisord`
+metadataTest:
+  exposedPorts: ["8888", "9000"]
+  volumes: ["/var/lib/snappymail"]
+  entrypoint: []
+  cmd: ["/entrypoint.sh"]
+  workdir: /snappymail
+  user: ''

.docker/release/test/test.sh

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+#!/bin/sh
+# This script tests a given docker image using https://github.com/GoogleContainerTools/container-structure-test
+set -eu
+
+IMAGE=${1:-snappymail/snappymail:test}
+SCRIPT_DIR=$( cd "$(dirname "$0")" && pwd )
+docker inspect "$IMAGE" > /dev/null 2>&1 || docker pull "$IMAGE" # Pull the image if it doesn't exist in docker daemon
+docker run --rm -i \
+    -v /var/run/docker.sock:/var/run/docker.sock:ro \
+    -v "$SCRIPT_DIR/config.yaml:/config.yaml:ro" \
+    gcr.io/gcp-runtimes/container-structure-test:latest test --image "$IMAGE" --config config.yaml