chore: add caching to Linux docker run

Add cache mount directives to Linux docker container so that reruns
are much quicker.

linux/Dockerfile

@@ -3,19 +3,23 @@
 # -----------
 FROM golang:1.21-alpine3.19 as build
 
-# Go build env
-ENV CGO_ENABLED=0
-
 WORKDIR /app
 
+# Go build env
+ENV CGO_ENABLED=0 \
+    GOCACHE=/go-cache \
+    GOMODCACHE=/gomod-cache
+
 # Install source deps
 COPY go.mod go.sum ./
-RUN go mod download
+RUN --mount=type=cache,target=/gomod-cache \
+    go mod download
 
 # Copy source & build
 COPY . .
-RUN go build -v -ldflags '-s' -o /bin/ryuk
-RUN chmod +x /bin/ryuk
+RUN --mount=type=cache,target=/gomod-cache \
+    --mount=type=cache,target=/go-cache \
+    go build -v -ldflags '-s' -o /bin/ryuk
 
 # -----------------
 # Distributed Image