Dockerfile fails more cleanly if .git is a submodule (#3591)

# Description of Changes

Our `Dockerfile` runs `COPY . .`. If we're in a submodule rather than a
"full" git repo, that causes `.git` to be a in a weird state inside the
docker container, where it thinks it's in a submodule even though the
parent directory context has been lost.

This PR just adds a clear error message early in the process, because
this has bitten us a few times.

# API and ABI breaking changes

None

# Expected complexity level and risk

1

# Testing

- [x] We appropriately get the failure if we try to `docker build` this
from within a submodule.

Co-authored-by: Zeke Foppa <bfops@users.noreply.github.com>

Dockerfile

@@ -4,6 +4,15 @@ FROM rust:bookworm AS builder
 WORKDIR /usr/src/app
 COPY . .
 
+# If we're in a git submodule, we'll have a corrupted/nonfunctional .git file instead of a proper .git directory.
+# To make the errors more sane, remove .git entirely.
+RUN if [ -f .git ]; then \
+      echo "❌ ERROR: .git is a file (likely a submodule pointer), not a directory." >&2; \
+      echo "This will cause errors in the build process, because git operations will fail." >&2; \
+      echo "To address this, replace the .git file with a proper .git directory." >&2; \
+      exit 1; \
+    fi
+
 RUN cargo build -p spacetimedb-standalone -p spacetimedb-cli --release --locked
 
 FROM rust:bookworm