guix: Passthrough BASE_CACHE into container

This allows depends-built packages to be cached.

contrib/guix/README.md

@@ -114,6 +114,12 @@ find output/ -type f -print0 | sort -z | xargs -r0 sha256sum
   depends tree. Setting this to the same directory across multiple builds of the
   depends tree can eliminate unnecessary redownloading of package sources.
 
+* _**BASE_CACHE**_
+
+  Set the depends tree cache for built packages. This is passed through to the
+  depends tree. Setting this to the same directory across multiple builds of the
+  depends tree can eliminate unnecessary building of packages.
+
 * _**MAX_JOBS**_
 
   Override the maximum number of jobs to run simultaneously, you might want to

contrib/guix/guix-build.sh

@@ -180,6 +180,11 @@ and untracked files and directories will be wiped, allowing you to start anew.
 EOF
 }
 
+# Create SOURCES_PATH and BASE_CACHE if they are non-empty so that we can map
+# them into the container
+[ -z "$SOURCES_PATH" ] || mkdir -p "$SOURCES_PATH"
+[ -z "$BASE_CACHE" ]   || mkdir -p "$BASE_CACHE"
+
 # Deterministically build Bitcoin Core
 # shellcheck disable=SC2153
 for host in $HOSTS; do
@@ -274,6 +279,7 @@ EOF
                                  --share="$OUTDIR"=/outdir \
                                  --expose="$(git rev-parse --git-common-dir)" \
                                  ${SOURCES_PATH:+--share="$SOURCES_PATH"} \
+                                 ${BASE_CACHE:+--share="$BASE_CACHE"} \
                                  --max-jobs="$MAX_JOBS" \
                                  ${SUBSTITUTE_URLS:+--substitute-urls="$SUBSTITUTE_URLS"} \
                                  ${ADDITIONAL_GUIX_COMMON_FLAGS} ${ADDITIONAL_GUIX_ENVIRONMENT_FLAGS} \
@@ -282,6 +288,7 @@ EOF
                                         SOURCE_DATE_EPOCH="${SOURCE_DATE_EPOCH:?unable to determine value}" \
                                         ${V:+V=1} \
                                         ${SOURCES_PATH:+SOURCES_PATH="$SOURCES_PATH"} \
+                                        ${BASE_CACHE:+BASE_CACHE="$BASE_CACHE"} \
                                         DISTSRC="$(DISTSRC_BASE=/distsrc-base && distsrc_for_host "$HOST")" \
                                         OUTDIR=/outdir \
                                       bash -c "cd /bitcoin && bash contrib/guix/libexec/build.sh"

contrib/guix/libexec/build.sh

@@ -167,6 +167,7 @@ esac
 make -C depends --jobs="$MAX_JOBS" HOST="$HOST" \
                                    ${V:+V=1} \
                                    ${SOURCES_PATH+SOURCES_PATH="$SOURCES_PATH"} \
+                                   ${BASE_CACHE+BASE_CACHE="$BASE_CACHE"} \
                                    i686_linux_CC=i686-linux-gnu-gcc \
                                    i686_linux_CXX=i686-linux-gnu-g++ \
                                    i686_linux_AR=i686-linux-gnu-ar \