chore(deps): bump alpine from 3.17.1 to 3.21.3 in /.github/ci (#1326)

Bumps alpine from 3.17.1 to 3.21.3.

---
updated-dependencies:
- dependency-name: alpine
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/ci/vdrproxy.dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.17.1 AS builder
+FROM alpine:3.21.3 AS builder
 
 ARG UID=1000
 ARG GID=1000
@@ -35,7 +35,7 @@ WORKDIR /home/indy/indy-vdr/indy-vdr-proxy
 RUN git checkout b4dc08b
 RUN cargo build --release
 
-FROM alpine:3.18
+FROM alpine:3.21
 RUN apk update && apk upgrade && \
     apk add --no-cache \
         libstdc++ \