Adds centos tests

.ci/docker/centos/Dockerfile

@@ -0,0 +1,19 @@
+FROM centos:8
+
+LABEL maintainer="mail@sobolevn.me"
+LABEL vendor="git-secret team"
+
+RUN dnf -y update \
+  && dnf install -y \
+    # Direct dependencies:
+    bash \
+    gawk \
+    git \
+    gnupg \
+    # Assumed to be present:
+    diffutils \
+    findutils \
+    procps \
+    make \
+  && dnf clean all \
+  && rm -rf /var/cache/yum

.github/workflows/test.yml

@@ -17,7 +17,7 @@ jobs:
           - ubuntu
           - alpine
           - fedora
-          # - centos
+          - centos
 
     steps:
     - uses: actions/checkout@v2