Remove Bash workaround for Alpine image

There was an issue with Bash >= 5.1 where the wait function did not
work as inteded and we had to limit which version we installed. This
[problem][1] is fixed as of [Bash 5.1.16][2], so we can remove our
special case again.

[1]: JonasAlfredsson/bash_fail-to-wait#2
[2]: https://git.savannah.gnu.org/cgit/bash.git/commit/?id=910fcdc415abeb3d7d85fb46ee0d3e804a4c47a6

Author: JonasAlfredsson

src/Dockerfile-alpine

@@ -21,6 +21,7 @@ ENV CERTBOT_DNS_AUTHENTICATORS \
 RUN set -ex && \
 # Install packages necessary during the build phase (for all architectures).
     apk add --no-cache \
+        bash \
         cargo \
         curl \
         findutils \
@@ -34,8 +35,6 @@ RUN set -ex && \
         python3-dev \
         sed \
     && \
-# Because of the bug mentioned in issue #42 we must use Bash <5.1.x for now.
-    apk add --no-cache --repository https://dl-cdn.alpinelinux.org/alpine/v3.12/main/ 'bash<5.1' && \
 # Install the latest version of PIP, Setuptools and Wheel.
     curl -L 'https://bootstrap.pypa.io/get-pip.py' | python3 && \
 # Install certbot.