chore: Dockerfile - Adopt HereDoc syntax

Dockerfile.alpine

@@ -1,8 +1,20 @@
+# syntax=docker.io/docker/dockerfile:1
+
 FROM alpine:3.21 AS base-alpine
-RUN apk add --no-cache bash procps drill coreutils libidn curl socat openssl xxd \
-    && addgroup testssl \
-    && adduser -G testssl -g "testssl user" -s /bin/bash -D testssl \
-    && ln -s /home/testssl/testssl.sh /usr/local/bin/testssl.sh
+RUN <<HEREDOC
+  apk add --no-cache bash procps drill coreutils libidn curl socat openssl xxd
+
+  # Add a non-root user `testssl`, this is roughly equivalent to the `adduser` command:
+  # addgroup testssl && adduser -G testssl -g "testssl user" -s /bin/bash -D testssl
+  echo 'testssl:x:1000:1000::/home/testssl:/bin/bash' >> /etc/passwd
+  echo 'testssl:x:1000:' >> /etc/group
+  echo 'testssl:!::0:::::' >> /etc/shadow
+  install --mode 2755 --owner testssl --group testssl --directory /home/testssl
+
+  # A copy of `testssl.sh` will be added to the home directory,
+  # symlink to that file so it can be treated as a command:
+  ln -s /home/testssl/testssl.sh /usr/local/bin/testssl.sh
+HEREDOC
 
 # Runtime config:
 USER testssl
@@ -10,7 +22,7 @@ ENTRYPOINT ["testssl.sh"]
 CMD ["--help"]
 
 # Final image stage (add `testssl.sh` project files)
-# Choose either one as the final stage (defaults to last stage, `dist-git`)
+# Choose either one as the final stage (defaults to the last stage, `dist-local`)
 
 # 35MB Image (Remote repo clone, cannot filter content through `.dockerignore`):
 FROM base-alpine AS dist-git