deps: update to Alpine 3.21 base images

Signed-off-by: Jacob Howard <[email protected]>

images/sidecar/linux/Dockerfile

@@ -1,5 +1,5 @@
 # Use an Alpine-based Go builder.
-FROM golang:1.23.6-alpine3.20 AS builder
+FROM golang:1.23.6-alpine3.21 AS builder
 
 # Disable cgo in order to match the behavior of our release binaries (and to
 # avoid the need for gcc on certain architectures).
@@ -21,7 +21,7 @@ RUN ["go", "build", "-o", "mutagen-agent-sspl", "-tags", "mutagenagent,mutagenss
 
 
 # Switch to a vanilla Alpine base for the final image.
-FROM alpine:3.20 AS base
+FROM alpine:3.21 AS base
 
 # Copy the sidecar entry point from the builder.
 COPY --from=builder ["/mutagen/mutagen-sidecar", "/usr/bin/mutagen-sidecar"]

scripts/ci/docker/linux/Dockerfile

@@ -1,5 +1,5 @@
 # Set the base image.
-FROM alpine:3.20
+FROM alpine:3.21
 
 # Add the HTTP demo server.
 COPY ["httpdemo", "/httpdemo"]