Fix up Dockerfiles

The Dockerfiles had bitrotted some.

Get them passing again, add libpcre2-dev where we can so we aren't
hitting more servers than necessary, and reformat the bionic files so
they can share more of the same image.

docker/bionic-tsan.Dockerfile

@@ -2,21 +2,23 @@ FROM ubuntu:18.04
 
 ENV LANG C.UTF-8
 ENV LC_ALL C.UTF-8
-ENV CXXFLAGS "-fsanitize=thread"
 
 RUN apt-get update \
   && apt-get -y install \
     build-essential \
     cmake \
+    clang-9 \
     gettext \
     git \
     libncurses5-dev \
+    libpcre2-dev \
     locales \
     ninja-build \
     python3 \
     python3-pexpect \
     sudo \
-  && locale-gen en_US.UTF-8
+  && locale-gen en_US.UTF-8 \
+  && apt-get clean
 
 RUN groupadd -g 1000 fishuser \
   && useradd -p $(openssl passwd -1 fish) -d /home/fishuser -m -u 1000 -g 1000 fishuser \
@@ -30,4 +32,5 @@ WORKDIR /home/fishuser
 
 COPY fish_run_tests.sh /
 
+ENV CXXFLAGS "-fsanitize=thread"
 CMD /fish_run_tests.sh