Cache Clojure classpath and prefill caches

For repos that are built with the [Clojure CLI tools][clojure-cli]
(i.e., repos marked with a `deps.edn` file), we cache and restore the
`.cpcache` directory. Clojure uses this directory to avoid unnecessarily
re-calculating the JVM classpath. For most projects this additional
cache will not lead to a large improvement in build times.

We also download dependencies to the `.m2` cache before handing off to
the build script. Technically, this is unnecessary since, presumably,
the build script uses the Clojure CLI tools which have a side-effect of
populating `.m2`. Therefore, the benefit of this change is simply to
separate the two steps, isolating any errors that occure while fetching
dependencies.

[clojure-cli]: https://clojure.org/reference/deps_and_cli

Dockerfile

@@ -376,6 +376,8 @@ RUN mkdir /opt/boot-clj && cd /opt/boot-clj && \
     chmod +x boot && \
     ln -s /opt/boot-clj/boot /usr/local/bin/boot
 
+# TODO: When upgrading to >= 1.10.1.672, also update approach for populating .m2 with clojure deps:
+# In run-build-functions.sh replace `clojure -Spath -Sforce >/dev/null` with more modern `clojure -P`
 RUN curl -sL https://download.clojure.org/install/linux-install-1.10.1.492.sh | bash
 
 USER buildbot

run-build-functions.sh

@@ -588,6 +588,15 @@ install_dependencies() {
   if [ -f deps.edn ]
   then
     restore_home_cache ".m2" "maven dependencies"
+    restore_cwd_cache ".cpcache" "clojure classpath"
+    echo "Installing Clojure dependencies"
+    if clojure -Spath -Sforce >/dev/null
+    then
+      echo "Clojure dependencies installed"
+    else
+      echo "Error during Clojure install"
+      exit 1
+    fi
   fi
 
   # Hugo
@@ -708,6 +717,7 @@ cache_artifacts() {
 
   cache_cwd_directory ".venv" "python virtualenv"
   cache_cwd_directory ".build" "swift build"
+  cache_cwd_directory ".cpcache" "clojure classpath"
   cache_cwd_directory ".netlify/plugins" "build plugins"
 
   if [ -f Cargo.toml ] || [ -f Cargo.lock ]