Adds ubuntu testing

.ci/docker/ubuntu/Dockerfile

@@ -0,0 +1,18 @@
+FROM ubuntu:20.04
+
+ENV DEBIAN_FRONTEND="noninteractive"
+
+RUN apt-get update \
+  && apt-get upgrade -y \
+  && apt-get install -y \
+    # Direct dependencies:
+    curl \
+    gawk \
+    git \
+    gnupg \
+    # Assumed to be present:
+    procps \
+    make \
+  # Cleaning cache:
+  && apt-get purge -y --auto-remove -o APT::AutoRemove::RecommendsImportant=false \
+  && apt-get clean -y && rm -rf /var/lib/apt/lists/*

.github/workflows/test.yml

@@ -14,7 +14,7 @@ jobs:
       matrix:
         docker-based-test:
           - debian
-          # - ubuntu
+          - ubuntu
           # - alpine
           # - fedora
           # - centos