Leverage Docker Layer System (#1191)

Dockerfile best practices recommend to move the installation of external dependencies before specific code.
This will improve local development experience by improving Docker cache management
and it will improve final Docker images compaction by sharing the first layers.

Dockerfile.alpine

@@ -1,9 +1,13 @@
 FROM alpine
 
 ARG TARGETARCH
-COPY ./dist/${TARGETARCH}/ttyd /usr/bin/ttyd
+
+# Dependencies
 RUN apk add --no-cache bash tini
 
+# Application
+COPY ./dist/${TARGETARCH}/ttyd /usr/bin/ttyd
+
 EXPOSE 7681
 WORKDIR /root