Remove GitHub app usage now that attest is public

Signed-off-by: Jonny Stoten <jonny.stoten@docker.com>

Dockerfile

@@ -20,24 +20,12 @@ ENV CGO_ENABLED=0
 
 WORKDIR /app
 
-# --- This block can be removed when github.com/docker/attest is public
-ENV GOPRIVATE="github.com/docker/attest"
-RUN --mount=type=secret,id=GITHUB_TOKEN <<EOT
-  set -e
-  GITHUB_TOKEN=${GITHUB_TOKEN:-$(cat /run/secrets/GITHUB_TOKEN)}
-  if [ -n "$GITHUB_TOKEN" ]; then
-    echo "Setting GitHub access token"
-    git config --global "url.https://x-access-token:${GITHUB_TOKEN}@github.com.insteadof" "https://github.com"
-  fi
-EOT
-# ---
-
 ARG VERSION="dev"
 
 RUN --mount=type=bind,source=.,target=/app \
-    --mount=type=cache,target=$GOPATH/pkg/mod \
-    --mount=type=cache,target=/root/.cache/go-build \
-    go build -ldflags "-X main.version=$VERSION" -o /bin/attest main.go
+  --mount=type=cache,target=$GOPATH/pkg/mod \
+  --mount=type=cache,target=/root/.cache/go-build \
+  go build -ldflags "-X main.version=$VERSION" -o /bin/attest main.go
 
 FROM ${BASEIMAGE} AS production