Turns out you just can't use build args in the entrypoint... why... who knows

.dockerignore

@@ -29,7 +29,6 @@
 /bin
 /target
 !/target/**/monzo-ingestion
-!/target/**/test-binary
 /.idea
 LICENSE
 README.md

Dockerfile

@@ -1,16 +1,14 @@
 FROM busybox AS platform_determiner
 ARG TARGETPLATFORM
 ARG BINARY_NAME
-WORKDIR /app
-COPY /target /app/target
+
+COPY /target /target
 RUN case "$TARGETPLATFORM" in \
-      "linux/amd64") BINARY_PATH="/app/target/release/${BINARY_NAME}" ;; \
+      "linux/amd64") BINARY_PATH="/target/release/${BINARY_NAME}" ;; \
       "linux/arm64") BINARY_PATH="/target/aarch64-unknown-linux-musl/release/${BINARY_NAME}" ;; \
       *) exit 1 ;; \
-    esac && \
-    mv "$BINARY_PATH" /usr/local/bin/${BINARY_NAME}
+    esac && mv "$BINARY_PATH" "/usr/bin/monzo-ingestion" && chmod +x "/usr/bin/monzo-ingestion"
 
 FROM --platform=$TARGETPLATFORM debian:bullseye-slim
-ARG BINARY_NAME
-COPY --from=platform_determiner /usr/local/bin/${BINARY_NAME} /usr/local/bin/${BINARY_NAME}
-ENTRYPOINT ["/usr/local/bin/${BINARY_NAME}"]
+COPY --from=platform_determiner /usr/bin/monzo-ingestion /usr/local/bin/monzo-ingestion
+ENTRYPOINT ["/usr/local/bin/monzo-ingestion"]