Update for V5.6

Author: knonomura

.github/workflows/dockerFile/Dockerfile-ubuntu

@@ -1,19 +1,11 @@
-FROM ubuntu:18.04
+FROM ubuntu:20.04
 
 # Enviroment
 ENV DEBIAN_FRONTEND noninteractive
 
 # Install dependency
 RUN set -eux \
     && apt-get update -y \
-    && apt-get install -y gcc-4.8 \
-    && apt-get install -y g++-4.8 \
-    && apt-get install -y debhelper libz-dev libsqlite3-dev default-jdk \
-                          default-jre ant tcl rsync zip python3 \
-                          systemd-sysv ubuntu-standard \
+    && apt-get install -y debhelper libz-dev default-jdk \
+                          ant tcl rsync zip python3 systemd \
     && apt-get clean all
-
-# Create softlink gcc g++
-RUN set -eux \
-    && ln -sf /usr/bin/g++-4.8 /usr/bin/g++ \
-    && ln -sf /usr/bin/gcc-4.8 /usr/bin/gcc \