Add dynamic recaptcha configuration in the Docker image

contrib/docker/README.md

@@ -86,6 +86,10 @@ Synapse specific settings:
 * ``SYNAPSE_EVENT_CACHE_SIZE``, the event cache size [default `10K`].
 * ``SYNAPSE_REPORT_STATS``, set this variable to `yes` to enable anonymous
   statistics reporting back to the Matrix project which helps us to get funding.
+* ``SYNAPSE_RECAPTCHA_PUBLIC_KEY``, set this variable to the recaptcha public
+  key in order to enable recaptcha upon registration
+* ``SYNAPSE_RECAPTCHA_PRIVATE_KEY``, set this variable to the recaptcha private
+  key in order to enable recaptcha upon registration
 
 Shared secrets, these will be initialized to random values if not set:
 

contrib/docker/conf/homeserver.yaml

@@ -112,10 +112,17 @@ max_spider_size: "10M"
 
 ## Captcha ##
 
+{% if SYNAPSE_RECAPTCHA_PUBLIC_KEY %}
+recaptcha_public_key: "{{ SYNAPSE_RECAPTCHA_PUBLIC_KEY }}"
+recaptcha_private_key: "{{ SYNAPSE_RECAPTCHA_PRIVATE_KEY }}"
+enable_registration_captcha: True
+recaptcha_siteverify_api: "https://www.google.com/recaptcha/api/siteverify"
+{% else %}
 recaptcha_public_key: "YOUR_PUBLIC_KEY"
 recaptcha_private_key: "YOUR_PRIVATE_KEY"
 enable_registration_captcha: False
 recaptcha_siteverify_api: "https://www.google.com/recaptcha/api/siteverify"
+{% endif %}
 
 ## Turn ##