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>

README.md

@@ -7,7 +7,7 @@
 Backup Docker volumes locally or to any S3 compatible storage.
 
 The [offen/docker-volume-backup](https://hub.docker.com/r/offen/docker-volume-backup) Docker image can be used as a lightweight (below 15MB) sidecar container to an existing Docker setup.
-It handles __recurring or one-off backups of Docker volumes__ to a __local directory__ or __any S3 compatible storage__ (or both), and __rotates away old backups__ if configured. It also supports __encrypting your backups using GPG__ and __sending notifications for failed backup runs__.
+It handles __recurring or one-off backups of Docker volumes__ to a __local directory__, __any S3 or WebDAV compatible storage (or any combination) and __rotates away old backups__ if configured. It also supports __encrypting your backups using GPG__ and __sending notifications for failed backup runs__.
 
 <!-- MarkdownTOC -->
 
@@ -28,6 +28,7 @@ It handles __recurring or one-off backups of Docker volumes__ to a __local direc
 - [Recipes](#recipes)
   - [Backing up to AWS S3](#backing-up-to-aws-s3)
   - [Backing up to MinIO](#backing-up-to-minio)
+  - [Backing up to WebDAV](#backing-up-to-webdav)
   - [Backing up locally](#backing-up-locally)
   - [Backing up to AWS S3 as well as locally](#backing-up-to-aws-s3-as-well-as-locally)
   - [Running on a custom cron schedule](#running-on-a-custom-cron-schedule)
@@ -189,6 +190,24 @@ You can populate below template according to your requirements and use it as you
 
 # AWS_ENDPOINT_INSECURE="true"
 
+# In addition, you can also backup files to any WebDAV server.
+# The URL of the remote WebDAV server
+
+# WEBDAV_URL="https://webdav.example.com"
+
+# The Directory to place the backups to on the WebDAV server.
+# If the path is not present on the server it will be created!
+
+# WEBDAV_PATH="/my/directory/"
+
+# The username for the WebDAV server
+
+# WEBDAV_USERNAME="user"
+
+# The password for the WebDAV server
+
+# WEBDAV_PASSWORD="password"
+
 # In addition to storing backups remotely, you can also keep local copies.
 # Pass a container-local path to store your backups if needed. You also need to
 # mount a local folder or Docker volume into that location (`/archive`
@@ -530,6 +549,28 @@ volumes:
   data:
 ```
 
+### Backing up to WebDAV
+
+```yml
+version: '3'
+
+services:
+  # ... define other services using the `data` volume here
+  backup:
+    image: offen/docker-volume-backup:latest
+    environment:
+      WEBDAV_URL: https://webdav.mydomain.me
+      WEBDAV_PATH: /my/directory/
+      WEBDAV_USERNAME: user
+      WEBDAV_PASSWORD: password
+    volumes:
+      - data:/backup/my-app-backup:ro
+      - /var/run/docker.sock:/var/run/docker.sock:ro
+
+volumes:
+  data:
+```
+
 ### Backing up locally
 
 ```yml