Calculate Docker cache hash precisely from Dockerfile's dependencies

`src/ci/docker`, so that when files under `dist-x86_64-linux` is changed,
its dependent image `dist-i686-linux` will also be rebuilt.

However, this ultraconservative solution caused the `dist-i686-linux` to
be rebuilt every time an irrelevant Dockerfile (e.g. the PowerPC ones) is
changed, which increases the building time beyond 3 hours and forcing
a spurious but expected failure.

This commit instead parses the Dockerfile itself and look for the actual
dependencies. The scripts needs to be copied into the Docker image, which
must be done with the COPY command, so we just need to find all lines with
a COPY command and add the source file into the hash calculator.

Note: this script only handles single-lined COPY command in the form
`COPY src1 src2 src3 dst`, since these are the only variant used inside
this repository.

src/ci/docker/run.sh

@@ -22,7 +22,18 @@ if [ -f "$docker_dir/$image/Dockerfile" ]; then
       hash_key=/tmp/.docker-hash-key.txt
       rm -f "${hash_key}"
       echo $image >> $hash_key
-      find $docker_dir -type f | sort | xargs cat >> $hash_key
+
+      cat "$docker_dir/$image/Dockerfile" >> $hash_key
+      # Look for all source files involves in the COPY command
+      copied_files=/tmp/.docker-copied-files.txt
+      rm -f "$copied_files"
+      for i in $(sed -n -e 's/^COPY \(.*\) .*$/\1/p' "$docker_dir/$image/Dockerfile"); do
+        # List the file names
+        find "$docker_dir/$i" -type f >> $copied_files
+      done
+      # Sort the file names and cat the content into the hash key
+      sort $copied_files | xargs cat >> $hash_key
+
       docker --version >> $hash_key
       cksum=$(sha512sum $hash_key | \
         awk '{print $1}')