Add ability to set timezone for Docker container

changelog.d/5383.feature

@@ -0,0 +1 @@
+Add ability to change Docker containers [timezone](https://en.wikipedia.org/wiki/List_of_tz_database_time_zones) with the `TZ` variable.

docker/Dockerfile

@@ -64,6 +64,7 @@ RUN apk add --no-cache --virtual .runtime_deps \
         libxslt \
         libpq \
         zlib \
+        tzdata \
         su-exec
 
 COPY --from=builder /install /usr/local

docker/README.md

@@ -86,6 +86,7 @@ Global settings:
 
 * ``UID``, the user id Synapse will run as [default 991]
 * ``GID``, the group id Synapse will run as [default 991]
+* ``TZ``, the [timezone](https://en.wikipedia.org/wiki/List_of_tz_database_time_zones) the container will run with [default UTC]
 * ``SYNAPSE_CONFIG_PATH``, path to a custom config file
 
 If ``SYNAPSE_CONFIG_PATH`` is set, you should generate a configuration file