Add ninja-build to all Dockerfiles

linux-fresh/Dockerfile

@@ -2,4 +2,4 @@ FROM ubuntu:18.04
 MAINTAINER citra
 RUN useradd -m -s /bin/bash citra
 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 libavformat-dev libswscale-dev wget git ccache cmake
+RUN apt-get install -y p7zip-full build-essential libsdl2-dev qtbase5-dev libqt5opengl5-dev qtmultimedia5-dev qttools5-dev qttools5-dev-tools libavcodec-dev libavformat-dev libswscale-dev wget git ccache cmake ninja-build

linux-frozen/Dockerfile

@@ -2,7 +2,7 @@ FROM ubuntu:18.04
 MAINTAINER citra
 RUN useradd -m -s /bin/bash citra && mkdir -p /tmp/pkgs
 COPY install_package.py /tmp/pkgs
-RUN apt-get update && apt-get install -y build-essential wget git python-launchpadlib ccache
+RUN apt-get update && apt-get install -y build-essential wget git python-launchpadlib ccache ninja-build
 RUN cd /tmp/pkgs && python2 install_package.py \
     libsdl2-dev 2.0.7+dfsg1-3ubuntu1 bionic          \
     qtbase5-dev 5.9.3+dfsg-0ubuntu2 bionic           \

linux-mingw/Dockerfile

@@ -1,7 +1,7 @@
 FROM ubuntu:18.04
 MAINTAINER citra
 RUN useradd -m -s /bin/bash citra && 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
+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
 # workaround broken headers in Ubuntu MinGW package
 COPY errno.h /usr/x86_64-w64-mingw32/include/
 # add mingw-w64 auxiliary ppa repository