Update Ubuntu version to 20.04 (#25)

* linux-clang-format: Update Ubuntu

* linux-flatpak: Update Ubuntu

* linux-fresh: Update Ubuntu

* linux-frozen: Update Ubuntu

* linux-mingw: Update Ubuntu

linux-clang-format/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:18.04
+FROM ubuntu:20.04
 LABEL maintainer="citraemu"
 RUN apt-get update && apt-get -y full-upgrade
 RUN apt-get install -y git clang-format-6.0 p7zip-full

linux-flatpak/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:18.04
+FROM ubuntu:20.04
 LABEL maintainer="citraemu"
 RUN apt-get update && apt-get -y full-upgrade
 RUN apt-get install -y p7zip-full wget git flatpak flatpak-builder ca-certificates sshfs curl fuse dnsutils gnupg2 sudo

linux-fresh/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:18.04
+FROM ubuntu:20.04
 LABEL maintainer="citraemu"
 RUN apt-get update && apt-get -y full-upgrade
 RUN apt-get install -y p7zip-full build-essential libsdl2-dev qtbase5-dev libqt5opengl5-dev qtmultimedia5-dev qttools5-dev qttools5-dev-tools libavcodec-dev libavfilter-dev libavformat-dev libswscale-dev wget git ccache cmake ninja-build

linux-frozen/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:18.04
+FROM ubuntu:20.04
 LABEL maintainer="citraemu"
 RUN mkdir -p /tmp/pkgs
 COPY install_package.py /tmp/pkgs

linux-mingw/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:18.04
+FROM ubuntu:20.04
 LABEL maintainer="citraemu"
 RUN mkdir -p /tmp/pkgs
 RUN apt-get update && apt-get install -y gpg wget git python3-pip ccache p7zip-full g++-mingw-w64-x86-64 gcc-mingw-w64-x86-64 mingw-w64-tools cmake ninja-build