POC: Introduce Nextcloud based on Nginx web server.

This PR introduce POC of Nextcloud based on Nginx web server with php-fpm in separated container.
Basic installation with SQLite and few operations in web gui were tested.

Nginx dockerfile is generated from Dockerfile-nginx.template
  - FROM string has hardcoded variant - apache
  - There is no upstream PHP image with Nginx, due this generate-stackbrew-library is not able to identify parent correctly

New files were introduced in image:
  - Dockerfile-nginx.template
  - nginx-foreground
    + simulates behavior of apache2-foreground cmd
  - 16.0/nginx/Dockerfile
    + generated from template Dockerfile-nginx.template
  - nextcloud.confx
    + Nginx vhost configuration
  - start_net.sh (temporary)
    + Helper script to create user-defined network for Docker
    + '--link' option for docker create is obsolete and we need communication between fpm and nginx container
  - start_container.sh (temporary)
    + Helper script to handle containers
  - Dockerfile
    + /Dockerfile for verification purposes
Updated files:
  - set -x added to all shell scripts for easier troubleshooting (temporary)
  - update.sh
    + New variant introduced
    + Number of builded variant is limited for simplified CI.
  - .travis.yml (temporary)
    + Image push to registry for testing purposes
  - docker-entrypoint.sh
    + Recognizes new argument for entrypoint - nginx-foreground

Be careful during review, github recognizes removed files for 15.0 release as renamed to 16.0/nginx variant

Motivation/Usecase:
  - I have openvz based VPS where only old Docker 1.10 is supported.
  - Apache(in nextcloud container) in combination Nginx as a reverse proxy for multiple vhosts doesn't work correctly

Image pull link:
docker pull 1john2ci/nextcloud:apache-20190509
docker pull 1john2ci/nextcloud:fpm-20190509
Travis CI test: 530329740

Deployment notes:
  - Clone source code
  - Create user-defined network for Docker
  - Start fpm container
  - Start nginx container

16.0/nginx/entrypoint.sh

@@ -0,0 +1,145 @@
+#!/bin/sh
+set -xeu
+
+# version_greater A B returns whether A > B
+version_greater() {
+    [ "$(printf '%s\n' "$@" | sort -t '.' -n -k1,1 -k2,2 -k3,3 -k4,4 | head -n 1)" != "$1" ]
+}
+
+# return true if specified directory is empty
+directory_empty() {
+    [ -z "$(ls -A "$1/")" ]
+}
+
+run_as() {
+    if [ "$(id -u)" = 0 ]; then
+        su -p www-data -s /bin/sh -c "$1"
+    else
+        sh -c "$1"
+    fi
+}
+
+# ##
+# Add new recognized entrycmd
+# ##
+if expr "$1" : "nginx" 1>/dev/null || expr "$1" : "apache" 1>/dev/null || [ "$1" = "php-fpm" ] || [ "${NEXTCLOUD_UPDATE:-0}" -eq 1 ]; then
+    if [ -n "${REDIS_HOST+x}" ]; then
+
+        echo "Configuring Redis as session handler"
+        {
+            echo 'session.save_handler = redis'
+            echo "session.save_path = \"tcp://${REDIS_HOST}:${REDIS_HOST_PORT:=6379}\""
+        } > /usr/local/etc/php/conf.d/redis-session.ini
+    fi
+
+    installed_version="0.0.0.0"
+    if [ -f /var/www/html/version.php ]; then
+        # shellcheck disable=SC2016
+        installed_version="$(php -r 'require "/var/www/html/version.php"; echo implode(".", $OC_Version);')"
+    fi
+    # shellcheck disable=SC2016
+    image_version="$(php -r 'require "/usr/src/nextcloud/version.php"; echo implode(".", $OC_Version);')"
+
+    if version_greater "$installed_version" "$image_version"; then
+        echo "Can't start Nextcloud because the version of the data ($installed_version) is higher than the docker image version ($image_version) and downgrading is not supported. Are you sure you have pulled the newest image version?"
+        exit 1
+    fi
+
+    if version_greater "$image_version" "$installed_version"; then
+        echo "Initializing nextcloud $image_version ..."
+        if [ "$installed_version" != "0.0.0.0" ]; then
+            echo "Upgrading nextcloud from $installed_version ..."
+            run_as 'php /var/www/html/occ app:list' | sed -n "/Enabled:/,/Disabled:/p" > /tmp/list_before
+        fi
+        if [ "$(id -u)" = 0 ]; then
+            rsync_options="-rlDog --chown www-data:root"
+        else
+            rsync_options="-rlD"
+        fi
+        rsync $rsync_options --delete --exclude-from=/upgrade.exclude /usr/src/nextcloud/ /var/www/html/
+
+        for dir in config data custom_apps themes; do
+            if [ ! -d "/var/www/html/$dir" ] || directory_empty "/var/www/html/$dir"; then
+                rsync $rsync_options --include "/$dir/" --exclude '/*' /usr/src/nextcloud/ /var/www/html/
+            fi
+        done
+        rsync $rsync_options --include '/version.php' --exclude '/*' /usr/src/nextcloud/ /var/www/html/
+        echo "Initializing finished"
+
+        #install
+        if [ "$installed_version" = "0.0.0.0" ]; then
+            echo "New nextcloud instance"
+
+            if [ -n "${NEXTCLOUD_ADMIN_USER+x}" ] && [ -n "${NEXTCLOUD_ADMIN_PASSWORD+x}" ]; then
+                # shellcheck disable=SC2016
+                install_options='-n --admin-user "$NEXTCLOUD_ADMIN_USER" --admin-pass "$NEXTCLOUD_ADMIN_PASSWORD"'
+                if [ -n "${NEXTCLOUD_TABLE_PREFIX+x}" ]; then
+                    # shellcheck disable=SC2016
+                    install_options=$install_options' --database-table-prefix "$NEXTCLOUD_TABLE_PREFIX"'
+                else
+                    install_options=$install_options' --database-table-prefix ""'
+                fi
+                if [ -n "${NEXTCLOUD_DATA_DIR+x}" ]; then
+                    # shellcheck disable=SC2016
+                    install_options=$install_options' --data-dir "$NEXTCLOUD_DATA_DIR"'
+                fi
+
+                install=false
+                if [  -n "${SQLITE_DATABASE+x}" ]; then
+                    echo "Installing with SQLite database"
+                    # shellcheck disable=SC2016
+                    install_options=$install_options' --database-name "$SQLITE_DATABASE"'
+                    install=true
+                elif [ -n "${MYSQL_DATABASE+x}" ] && [ -n "${MYSQL_USER+x}" ] && [ -n "${MYSQL_PASSWORD+x}" ] && [ -n "${MYSQL_HOST+x}" ]; then
+                    echo "Installing with MySQL database"
+                    # shellcheck disable=SC2016
+                    install_options=$install_options' --database mysql --database-name "$MYSQL_DATABASE" --database-user "$MYSQL_USER" --database-pass "$MYSQL_PASSWORD" --database-host "$MYSQL_HOST"'
+                    install=true
+                elif [ -n "${POSTGRES_DB+x}" ] && [ -n "${POSTGRES_USER+x}" ] && [ -n "${POSTGRES_PASSWORD+x}" ] && [ -n "${POSTGRES_HOST+x}" ]; then
+                    echo "Installing with PostgreSQL database"
+                    # shellcheck disable=SC2016
+                    install_options=$install_options' --database pgsql --database-name "$POSTGRES_DB" --database-user "$POSTGRES_USER" --database-pass "$POSTGRES_PASSWORD" --database-host "$POSTGRES_HOST"'
+                    install=true
+                fi
+
+                if [ "$install" = true ]; then
+                    echo "starting nextcloud installation"
+                    max_retries=10
+                    try=0
+                    until run_as "php /var/www/html/occ maintenance:install $install_options" || [ "$try" -gt "$max_retries" ]
+                    do
+                        echo "retrying install..."
+                        try=$((try+1))
+                        sleep 3s
+                    done
+                    if [ "$try" -gt "$max_retries" ]; then
+                        echo "installing of nextcloud failed!"
+                        exit 1
+                    fi
+                    if [ -n "${NEXTCLOUD_TRUSTED_DOMAINS+x}" ]; then
+                        echo "setting trusted domains…"
+                        NC_TRUSTED_DOMAIN_IDX=1
+                        for DOMAIN in $NEXTCLOUD_TRUSTED_DOMAINS ; do
+                            DOMAIN=$(echo "$DOMAIN" | sed -e 's/^[[:space:]]*//' -e 's/[[:space:]]*$//')
+                            run_as "php /var/www/html/occ config:system:set trusted_domains $NC_TRUSTED_DOMAIN_IDX --value=$DOMAIN"
+                            NC_TRUSTED_DOMAIN_IDX=$(($NC_TRUSTED_DOMAIN_IDX+1))
+                        done
+                    fi
+                else
+                    echo "running web-based installer on first connect!"
+                fi
+            fi
+        #upgrade
+        else
+            run_as 'php /var/www/html/occ upgrade'
+
+            run_as 'php /var/www/html/occ app:list' | sed -n "/Enabled:/,/Disabled:/p" > /tmp/list_after
+            echo "The following apps have been disabled:"
+            diff /tmp/list_before /tmp/list_after | grep '<' | cut -d- -f2 | cut -d: -f1
+            rm -f /tmp/list_before /tmp/list_after
+
+        fi
+    fi
+fi
+
+exec "$@"