Support Nextcloud / WebDav (#48)

* add studio-b12/gowebdav to be able to upload to webdav server

* make sure all env variables are present for webdav upload

* implement file upload to WebDav server

directory defaults to the base directory

* docs: add the new feature to the documentation

* if no WebDav env variable are given throw no error

* docs: use more elegant english :D

Co-authored-by: Frederik Ring <frederik.ring@gmail.com>

* docs: use official spelling of "WebDAV"

* perf: golang likes to return early instead of having an else block

* use WEBDAV_PATH instead of WEBDAV_DIRECTORY

* use split_words for more convenience

like shown here: https://github.com/kelseyhightower/envconfig#struct-tag-support

* simplify

* feat: add pruning of files in WebDAV remote

Based on / Inspired by the minio/S3 implementation of pruning remote files.

* remove logging from the development

* test: first try implementing tests

Sandly I have to use the remote pipeline -- local wont work for me.

* test: adapt used volume names

* test: specify image only once!

* test: minio AND webdav data should be present

* test: backups on WebDAV remote are laying in the root directory

* test: the webdav server stores date in /var/lib/dav

* trying with data subfolder

* test: 1 container was added so the number raised from 3 to 4

* webdav  subfolder is "data" not "backup"

* fix: password AND username must be defined

not password OR username

* improve logging

* feat: if the given path on the server isnt preset it will be created

* test: add creation of new folder for webdav to tests

Co-authored-by: Frederik Ring <frederik.ring@gmail.com>

test/compose/docker-compose.yml

@@ -10,13 +10,23 @@ services:
       MINIO_SECRET_KEY: GMusLtUmILge2by+z890kQ
     entrypoint: /bin/ash -c 'mkdir -p /data/backup && minio server /data'
     volumes:
-      - backup_data:/data
+      - minio_backup_data:/data
+
+  webdav:
+    image: bytemark/webdav:2.4
+    environment:
+      AUTH_TYPE: Digest
+      USERNAME: test
+      PASSWORD: test
+    volumes:
+      - webdav_backup_data:/var/lib/dav
 
   backup: &default_backup_service
     image: offen/docker-volume-backup:${TEST_VERSION}
     hostname: hostnametoken
     depends_on:
       - minio
+      - webdav
     restart: always
     environment:
       AWS_ACCESS_KEY_ID: test
@@ -32,6 +42,10 @@ services:
       BACKUP_PRUNING_LEEWAY: 5s
       BACKUP_PRUNING_PREFIX: test
       GPG_PASSPHRASE: 1234secret
+      WEBDAV_URL: http://webdav/
+      WEBDAV_PATH: /my/new/path/
+      WEBDAV_USERNAME: test
+      WEBDAV_PASSWORD: test
     volumes:
       - ./local:/archive
       - app_data:/backup/app_data:ro
@@ -45,5 +59,6 @@ services:
       - app_data:/var/opt/offen
 
 volumes:
-  backup_data:
+  minio_backup_data:
+  webdav_backup_data:
   app_data: