fix(ci): copy testapp go.mod/go.sum before download, fix hadolint warnings

Also copy apps/testapp/go.mod and apps/testapp/go.sum before running
go mod download so the testapp's own dependencies are cached in their
own layer (separate from the root module).

Replace RUN cd ... && go install with WORKDIR + go build to resolve
hadolint DL3003 and DL3062.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: tac0turtle

apps/testapp/Dockerfile

@@ -20,11 +20,14 @@ WORKDIR /ev-node
 # Copy module files first to leverage Docker layer caching.
 # Dependencies are only re-downloaded when go.mod or go.sum change.
 COPY go.mod go.sum ./
-RUN go mod download
+COPY apps/testapp/go.mod apps/testapp/go.sum ./apps/testapp/
+RUN go mod download && (cd apps/testapp && go mod download)
 
 # Copy the rest of the source and build.
 COPY . .
-RUN cd apps/testapp && go install .
+
+WORKDIR /ev-node/apps/testapp
+RUN go build -o /go/bin/testapp .
 
 ## prep the final image.
 #