[Heartbeat] Add default TZ to heartbeat docker images (elastic#36193)

## What does this PR do?

Fixes elastic#36117. Add default TZ
value to fix synthetic journeys for timezone-triggered errors.

## Why is it important?

Unspecified timezone values can cause navigation errors on synthetics
journeys.

3# How to test this PR locally

1. Build heartbeat docker image locally.
2. Create a journey that navigates to a Kibana URL.
3. Wait for Kibana login page to load properly.

CHANGELOG.next.asciidoc

@@ -136,6 +136,7 @@ automatic splitting at root level, if root level element is an array. {pull}3415
 - Fix temp dir running out of space with project monitors. {issue}35843[35843]
 - Fixing the grok expression outputs of log files {pull}35221[35221]
 - Enable heartbeat-wide publish timeout setting with run_once. {pull}35721[35721]
+- Added default timezone UTC to heartbeat docker images to fix synthetics journeys navigation errors. {pull}36193[36193]
 
 *Heartbeat*
 

dev-tools/packaging/templates/docker/Dockerfile.tmpl

@@ -121,6 +121,7 @@ RUN echo \
 
 # Setup synthetics env vars
 ENV ELASTIC_SYNTHETICS_CAPABLE=true
+ENV TZ=UTC
 ENV SUITES_DIR={{ $beatHome }}/suites
 ENV NODE_VERSION=18.16.0
 ENV PATH="$NODE_PATH/node/bin:$PATH"