docker: remove tzdata interactive dialog workaround

Instead set an environment variable to tell apt not to ask questions.

taskcluster/docker/base/Dockerfile

@@ -20,16 +20,14 @@ WORKDIR /builds/worker/
 ENV CURL='curl --location --retry 5' \
     GRADLE_OPTS='-Xmx4096m -Dorg.gradle.daemon=false' \
     LANG='en_US.UTF-8' \
-    TERM='dumb'
+    TERM='dumb' \
+    DEBIAN_FRONTEND=noninteractive
 
 #----------------------------------------------------------------------------------------------------------------------
 #-- System ------------------------------------------------------------------------------------------------------------
 #----------------------------------------------------------------------------------------------------------------------
 
 RUN apt-get update -qq \
-    # We need to install tzdata before all of the other packages. Otherwise it will show an interactive dialog that
-    # we cannot navigate while building the Docker image.
-    && apt-get install -y tzdata \
     && apt-get install -y openjdk-8-jdk \
                           openjdk-11-jdk \
                           wget \
@@ -41,6 +39,7 @@ RUN apt-get update -qq \
                           python3 \
                           python3-yaml \
                           locales \
+                          tzdata \
                           unzip \
 			  mercurial \
     && apt-get clean