fix: robust coverage calculation in test suite script

Fix bash arithmetic syntax error when calculating coverage percentages
from LCOV files. The issue occurred when grep results contained whitespace
or when values were empty strings.

Dockerfile

@@ -12,6 +12,9 @@ COPY webapp/package*.json ./
 RUN --mount=type=cache,target=/root/.npm \
     npm ci --no-audit --no-fund --no-progress
 COPY webapp/ ./
+# Enable code instrumentation for E2E coverage if requested
+ARG CYPRESS_COVERAGE=false
+ENV CYPRESS_COVERAGE=$CYPRESS_COVERAGE
 RUN npm run build
 
 FROM golang:alpine AS builder