refactor: Change base image from alpine:3.17 to opensuse/leap:15.4

Dockerfile

@@ -1,11 +1,87 @@
-FROM alpine:3.17
+# syntax=docker.io/docker/dockerfile:1
+# HereDoc (EOF) feature (avoids needing `&& \`) requires BuildKit:
+# https://docs.docker.com/engine/reference/builder/#here-documents
+ARG LEAP_VERSION=15.4
+ARG CACHE_ZYPPER=/tmp/cache/zypper
+ARG INSTALL_ROOT=/rootfs
 
-RUN apk update && \
-    apk upgrade && \
-    apk add bash procps drill git coreutils libidn curl socat openssl xxd && \
-    rm -rf /var/cache/apk/* && \
-    adduser -D -s /bin/bash testssl && \
-    ln -s /home/testssl/testssl.sh /usr/local/bin/ 
+FROM opensuse/leap:${LEAP_VERSION} as builder
+ARG CACHE_ZYPPER
+ARG INSTALL_ROOT
+# --mount is only necessary for persisting the zypper cache on the build host,
+# Paired with --cache-dir below, RUN layer invalidation does not clear this cache.
+# Not useful for CI, only local builds that retain the storage.
+RUN --mount=type=cache,target="${CACHE_ZYPPER}",sharing=locked <<EOF
+  INSTALL_DEPS=()
+
+  # Mandatory commands. coreutils required over busybox for date command:
+  # https://github.com/drwetter/testssl.sh/commit/d1f03801738c87b6af39372c45e048af78c73c09
+  INSTALL_DEPS+=(bash procps grep gawk sed coreutils)
+
+  # Support better performance and debugging than hexdump via xxd:
+  # https://github.com/drwetter/testssl.sh/pull/1862
+  # busybox-util-linux (mandatory: hexdump) + busybox-vi (optional: xxd)
+  INSTALL_DEPS+=( busybox-util-linux busybox-vi )
+
+  # Support IDN (Internationalized Domain Names) lookups with drill:
+  # https://github.com/drwetter/testssl.sh/pull/1326
+  INSTALL_DEPS+=( ldns libidn2-0 )
+
+  # Support StartTLS injection:
+  # https://github.com/drwetter/testssl.sh/pull/1810
+  INSTALL_DEPS+=( socat openssl )
+
+  # Support --phone-out checks:
+  # https://github.com/drwetter/testssl.sh/commit/a66f5cfdbcd93427f4408bdd8cfc336488c02bb8
+  INSTALL_DEPS+=( curl )
+
+
+  # Provides $VERSION_ID
+  source /etc/os-release
+
+  # --releasever required due to no version info in install root.
+  # --installroot installs to location as if it was the system root.
+  # --cache-dir with above `RUN --mount` speeds this step up.
+  ZYPPER_OPTIONS=(
+    --releasever "${VERSION_ID}"
+    --installroot "${INSTALL_ROOT}"
+    --cache-dir "${CACHE_ZYPPER}"
+  )
+
+  # Sync package repos to get latest updates:
+  zypper ${ZYPPER_OPTIONS[@]} --gpg-auto-import-keys refresh
+
+  zypper ${ZYPPER_OPTIONS[@]} --non-interactive install \
+    --download-in-advance --no-recommends ${INSTALL_DEPS[@]}
+
+
+  # Clears the cache, but this is not stored in the install root location (like DNF does), thus not useful.
+  # zypper ${ZYPPER_OPTIONS[@]} clean --all
+
+  # Unlike DNF, there isn't a `--nodocs` install option, manually remove some excess weight (9 MiB):
+  rm -r "${INSTALL_ROOT}/usr/share/"{licenses,man,locale,doc,help,info}
+  # Neither of these should be needed in the container, removes 4MiB
+  rm "${INSTALL_ROOT}/usr/share/misc/termcap"
+  rm -r "${INSTALL_ROOT}/usr/lib/sysimage/rpm"
+EOF
+
+
+# Create a new image with the contents of $INSTALL_ROOT
+FROM scratch
+ARG INSTALL_ROOT
+COPY --link --from=builder ${INSTALL_ROOT} /
+
+# zypper package `busybox-adduser` fails to install with `--installroot`,
+# while the `shadow` package is too heavy just to add a user.
+#
+# Temporarily bind mount the `/bin` dir from another image that already
+# has the `adduser` command, and it'll update `/etc/{group,passwd,shadow}` for us:
+# Absolute path provided as some base images PATH would use those binaries instead,
+# `adduser` varies in supported args, so this should avoid any surprises:
+RUN --mount=type=bind,from=busybox:latest,source=/bin,target=/bin <<EOF
+  /bin/adduser -D -s /bin/bash testssl
+  /bin/ln -s /home/testssl/testssl.sh /usr/local/bin/
+EOF
 
 USER testssl
 WORKDIR /home/testssl/
@@ -14,5 +90,4 @@ WORKDIR /home/testssl/
 COPY --chown=testssl:testssl . /home/testssl/
 
 ENTRYPOINT ["testssl.sh"]
-
 CMD ["--help"]