[docker] Bump Docker image to Ubuntu 20.04

Signed-off-by: Miles Dai <milesdai@google.com>
diff --git a/util/container/Dockerfile b/util/container/Dockerfile
index f974be5..b76dbd7 100644
--- a/util/container/Dockerfile
+++ b/util/container/Dockerfile
@@ -18,7 +18,7 @@
 ARG GCC_VERSION=9
 
 # Main container image.
-FROM ubuntu:18.04 AS opentitan
+FROM ubuntu:20.04 AS opentitan
 ARG VERILATOR_VERSION
 ARG OPENOCD_VERSION
 ARG VERIBLE_VERSION
@@ -36,7 +36,7 @@
 
 # Add OBS repository to apt sources.
 RUN OBS_URL="https://download.opensuse.org/repositories"; \
-    OBS_PATH="/home:/phiwag:/edatools/xUbuntu_18.04"; \
+    OBS_PATH="/home:/phiwag:/edatools/xUbuntu_20.04"; \
     REPO_URL="${OBS_URL}${OBS_PATH}"; \
     \
     EDATOOLS_REPO_KEY="${REPO_URL}/Release.key"; \
@@ -63,6 +63,11 @@
 # - minicom and screen are useful to see UART communication.
 # - dc and time are requirements of Synopsys VCS.
 # - software-properties-common is required to be able to install newer gcc versions.
+
+# Necessary to avoid user interaction with tzdata during install
+ARG DEBIAN_FRONTEND=noninteractive
+ENV TZ=UTC
+
 COPY apt-requirements.txt /tmp/apt-requirements.txt
 RUN echo "verilator-${VERILATOR_VERSION}" >>/tmp/apt-requirements.txt \
     && echo "openocd-${OPENOCD_VERSION}"  >>/tmp/apt-requirements.txt \