In `@docker/Dockerfile`:
- Around line 109-111: The Dockerfile uses the shell variable
BUILD_CUSTOM_FLASHINFER in the conditional block but never declares it as a
build ARG, so add an ARG declaration (e.g., add "ARG BUILD_CUSTOM_FLASHINFER" or
"ARG BUILD_CUSTOM_FLASHINFER=0") above the if-statement to ensure the build-arg
is accepted and propagated; keep the existing conditional that calls
tools/build-custom-flashinfer.sh when BUILD_CUSTOM_FLASHINFER is non-empty so
builds can enable this via --build-arg BUILD_CUSTOM_FLASHINFER=1.

In `@tools/build-custom-flashinfer.sh`:
- Around line 38-41: The script currently forces SSH host key verification off
via GIT_SSH_COMMAND which is insecure; change it to only disable verification
when an explicit opt-in env var is set (e.g. ALLOW_INSECURE_SSH=true or
INTERNAL_BUILD=true) and only for SSH-style URLs (check GIT_URL starts with
"ssh://" or matches an SSH host:path pattern) before setting GIT_SSH_COMMAND;
otherwise run a normal git clone to preserve host key checking. Update the logic
around GIT_SSH_COMMAND, GIT_URL and BUILD_DIR so the insecure options are
conditional on the env var and SSH URL detection.
- Around line 58-79: The script fails because project is never defined and it
blindly appends into vllm causing duplicates; fix by initializing project =
doc.setdefault("project", {}) then opt =
project.setdefault("optional-dependencies", {}) and vllm_list =
opt.setdefault("vllm", []) and only append "flashinfer-python" if it's not
already present; also make the sources addition idempotent by checking
sources.get("flashinfer-python") and only set it when missing or different (use
the existing symbols pyproject_path, doc, project, opt, vllm_list, sources, and
the "flashinfer-python" key).
- Around line 26-30: The script currently runs realpath on a target that may not
exist, causing the script to abort under set -e; update the BUILD_DIR assignment
so it does not call realpath on a non-existent path (either use
BUILD_DIR="$SCRIPT_DIR/../3rdparty/flashinfer" or use realpath -m
"$SCRIPT_DIR/../3rdparty/flashinfer") and keep the subsequent existence check
that echoes the error and exits; modify the line that defines BUILD_DIR (and any
usages relying on its absolute value) to use one of these safer alternatives.