Use Alpine 3.18 in Dockerfile

Alpine 3.19 was recently release: https://www.alpinelinux.org/posts/Alpine-3.19.0-released.html
and it does not have libssl1.1 anymore

We should start to use OpenSSL 3 but doing this so main branch isn't
broken. Reported in #141.

Dockerfile

@@ -5,7 +5,7 @@ RUN shards install --production
 COPY src/ src/
 RUN shards build --production --release --debug
 
-FROM alpine:latest
+FROM alpine:3.18
 RUN apk add --no-cache libssl1.1 pcre2 libevent libgcc \
     && addgroup --gid 1000 amqpproxy \
     && adduser --no-create-home --disabled-password --uid 1000 amqpproxy -G amqpproxy