Drops the apt-installed shellcheck in favor of the dotslash wrapper for version parity with the host. Bootstraps dotslash with arch detection so the Dockerfile builds on both aarch64 and x86_64.

Assisted-by: Claude Opus 4.7 via Claude Code

diff --git a/.ramekin/Dockerfile b/.ramekin/Dockerfile
index 6e8e5803..fa970999 100644
--- a/.ramekin/Dockerfile
+++ b/.ramekin/Dockerfile
@@ -1,8 +1,12 @@
 FROM ramekin-agent
 
-RUN apt-get update && apt-get install -y --no-install-recommends \
-    shellcheck \
-    && rm -rf /var/lib/apt/lists/*
-
-RUN curl -fsSL https://github.com/facebook/dotslash/releases/download/v0.5.8/dotslash-linux-musl.x86_64.tar.gz \
+RUN ARCH=$(uname -m) && \
+    case "$ARCH" in \
+        aarch64) DS_ARCH="aarch64" ;; \
+        x86_64) DS_ARCH="x86_64" ;; \
+        *) echo "Unsupported architecture: $ARCH" && exit 1 ;; \
+    esac && \
+    curl -fsSL "https://github.com/facebook/dotslash/releases/download/v0.5.8/dotslash-linux-musl.${DS_ARCH}.tar.gz" \
     | tar xz -C /usr/local/bin dotslash
+
+COPY bin/shellcheck /usr/local/bin/shellcheck