Add CI build for Ubuntu 18.04 LTS

This is a popular old server distro, let's make sure it continues to
work.

.dockerignore

@@ -1,3 +1,5 @@
 build*
 out/*
+.git/*
+.vs/*
 .vscode/*

.github/workflows/linux.yml

@@ -35,16 +35,6 @@ jobs:
       with:
         name: Odamex-Linux-x86_64
         path: 'build/artifact/*'
-  build-centos:
-    name: Build (CentOS)
-    needs: pre_job
-    if: ${{ needs.pre_job.outputs.should_skip != 'true' }}
-    runs-on: ubuntu-latest
-    steps:
-    - name: Checkout source
-      uses: actions/checkout@v2
-    - name: Run build
-      run: bash ci/centos-buildgen.sh
   build-sdl12:
     name: Build (SDL 1.2)
     needs: pre_job
@@ -74,3 +64,23 @@ jobs:
       run: bash ci/ubuntu-buildgen.sh
     - name: Run build
       run: cmake --build ./build/
+  build-centos:
+    name: Build (CentOS 7)
+    needs: pre_job
+    if: ${{ needs.pre_job.outputs.should_skip != 'true' }}
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout source
+      uses: actions/checkout@v2
+    - name: Run build
+      run: bash ci/centos-buildgen.sh
+  build-ubuntu-bionic:
+    name: Build (Ubuntu 18.04 LTS)
+    needs: pre_job
+    if: ${{ needs.pre_job.outputs.should_skip != 'true' }}
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout source
+      uses: actions/checkout@v2
+    - name: Run build
+      run: bash ci/ubuntu-bionic-buildgen.sh

ci/ubuntu-bionic-buildgen.sh

@@ -0,0 +1,11 @@
+#!/usr/bin/env bash
+
+# http://redsymbol.net/articles/unofficial-bash-strict-mode/
+
+set -euo pipefail
+IFS=$'\n\t'
+
+set -x
+
+docker build -t odamex -f ci/ubuntu-bionic.Dockerfile .
+docker run --rm odamex

ci/ubuntu-bionic.Dockerfile

@@ -0,0 +1,25 @@
+FROM ubuntu:18.04
+
+WORKDIR odamex
+
+COPY . .
+
+# Packages - first the majority of them, then cmake
+RUN set -x && \
+    apt update && \
+    apt install -y ninja-build libsdl2-dev libsdl2-mixer-dev \
+        libpng-dev libcurl4-openssl-dev libwxgtk3.0-gtk3-dev deutex \
+        apt-transport-https ca-certificates gnupg software-properties-common wget && \
+    wget -O - 'https://apt.kitware.com/keys/kitware-archive-latest.asc' 2>/dev/null | gpg --dearmor - | tee /etc/apt/trusted.gpg.d/kitware.gpg >/dev/null && \
+    apt-add-repository 'deb https://apt.kitware.com/ubuntu/ bionic main' && \
+    apt update && apt install -y cmake
+
+WORKDIR build
+
+# Build commands
+RUN cmake .. -GNinja \
+    -DCMAKE_BUILD_TYPE=RelWithDebInfo \
+    -DBUILD_OR_FAIL=1 -DBUILD_CLIENT=1 -DBUILD_SERVER=1 \
+    -DBUILD_MASTER=1 -DBUILD_LAUNCHER=1
+
+CMD ["ninja"]