feat: use S3 buildcache

Part of !410 (closed), factorizing. This doesn't write to S3, just reads from S3.

containers/jug/dev.Dockerfile

@@ -17,6 +17,7 @@ RUN --mount=type=cache,target=/var/cache/apt                            \
         python3                                                         \
         python3-dev                                                     \
         python3-distutils                                               \
+        python3-boto3                                                   \
         python-is-python3                                               \
  && rm -rf /var/lib/apt/lists/*
 
@@ -66,6 +67,20 @@ RUN --mount=type=cache,target=/var/cache/spack-mirror                   \
  && spack mirror add docker /var/cache/spack-mirror                     \
  && spack mirror list
 
+## Setup eic-spack buildcache mirrors (FIXME: leaks credentials into layer)
+ARG S3_ACCESS_KEY=""
+ARG S3_SECRET_KEY=""
+RUN --mount=type=cache,target=/var/cache/spack-mirror                   \
+    export PATH=$PATH:$SPACK_ROOT/bin                                   \
+ && if [ -n $S3_ACCESS_KEY ] ; then                                     \
+    spack mirror add --scope site                                       \
+      --s3-endpoint-url https://dtn01.sdcc.bnl.gov:9000                 \
+      --s3-access-key-id ${S3_ACCESS_KEY}                               \
+      --s3-access-key-secret ${S3_SECRET_KEY}                           \
+      eic-spack s3://eictest/EPIC/spack                                 \
+    ; fi                                                                \
+ && spack mirror list
+
 ## This variable will change whenevery either spack.yaml or our spack package
 ## overrides change, triggering a rebuild
 ARG CACHE_BUST="hash"