fix(ci): remove insecure git clone flag, add ca-certificates

Remove -c http.sslverify=false from all git clones
(enables proper TLS verification, closes MITM risk).
Explicitly install ca-certificates in every CI Docker image.

Dockerfile.debian

@@ -9,6 +9,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   autoconf \
   autoconf-archive \
   automake \
+  ca-certificates \
   ccache \
   gcc \
   git \
@@ -40,8 +41,8 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
 RUN mkdir -p /usr/src/{stabber,libstrophe,profanity}
 WORKDIR /usr/src
 
-RUN git clone --depth 1 -c http.sslverify=false https://git.jabber.space/devs/stabber
-RUN git clone --depth 1 -c http.sslverify=false https://github.com/strophe/libstrophe
+RUN git clone --depth 1 https://git.jabber.space/devs/stabber
+RUN git clone --depth 1 https://github.com/strophe/libstrophe
 
 WORKDIR /usr/src/stabber
 RUN ./bootstrap.sh