Watchtower HTTP API based updates (#432)

* Added HTTP API trigger to update running images

* Adds HTTP API authentication token parameter and handling

* Exposes port 8080 in Dockerfile to allow inter-container update triggering via HTTP API

* Fixes codacy issue

* Adds API usage doc

* Fix grammar

* Moves api logic to a package of its own

* Makes WT exit if token has not been set in HTTP API mode

* Adds lock to prevent concurrent updates when in HTTP API mode

Co-authored-by: Simon Aronsson <simme@arcticbit.se>

docs/arguments.md

@@ -193,6 +193,26 @@ Run an update attempt against a container name list one time immediately and exi
 Environment Variable: WATCHTOWER_RUN_ONCE
                 Type: Boolean
              Default: false
+```
+
+## HTTP API Mode
+Runs Watchtower in HTTP API mode, only allowing image updates to be triggered by an HTTP request.
+
+```
+            Argument: --http-api
+Environment Variable: WATCHTOWER_HTTP_API
+                Type: Boolean
+             Default: false
+``` 
+
+## HTTP API Token
+Sets an authentication token to HTTP API requests.
+
+```
+            Argument: --http-api-token
+Environment Variable: WATCHTOWER_HTTP_API_TOKEN
+                Type: String
+             Default: -
 ``` 
 
 ## Scheduling

docs/http-api-mode.md

@@ -0,0 +1,35 @@
+Watchtower provides an HTTP API mode that enables an HTTP endpoint that can be requested to trigger container updating. The current available endpoint list is:
+
+- `/v1/update` - triggers an update for all of the containers monitored by this Watchtower instance.
+
+---
+
+To enable this mode, use the flag `--http-api`. For example, in a Docker Compose config file:
+
+```json
+version: '3'
+
+services:
+  app-monitored-by-watchtower:
+    image: myapps/monitored-by-watchtower
+    labels:
+      - "com.centurylinklabs.watchtower.enable=true"
+
+  watchtower:
+    image: containrrr/watchtower
+    volumes:
+      - /var/run/docker.sock:/var/run/docker.sock
+    command: --debug --http-api
+    environment:
+      - WATCHTOWER_HTTP_API_TOKEN=mytoken
+    labels:
+      - "com.centurylinklabs.watchtower.enable=false"
+    ports:
+      - 8080:8080
+```
+
+Notice that there is an environment variable named WATCHTOWER_HTTP_API_TOKEN. To prevent external services from accidentally triggering image updates, all of the requests have to contain a "Token" field, valued as the token defined in WATCHTOWER_HTTP_API_TOKEN, in their headers. In this case, there is a port bind to the host machine, allowing to request localhost:8080 to reach Watchtower. The following `curl` command would trigger an image update:
+
+```bash
+curl -H "Token: mytoken" localhost:8080/v1/update
+```