Bump alpine from 3.20 to 3.21 in /docker (#5288)

Bumps alpine from 3.20 to 3.21.

---
updated-dependencies:
- dependency-name: alpine
  dependency-version: '3.21'
  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>

docker/Dockerfile

@@ -32,7 +32,7 @@ RUN --mount=type=cache,target=/root/.cache/crystal if [[ "${release}" == 1 ]] ;
         --link-flags "-lxml2 -llzma"; \
     fi
 
-FROM alpine:3.20
+FROM alpine:3.21
 RUN apk add --no-cache rsvg-convert ttf-opensans tini tzdata
 WORKDIR /invidious
 RUN addgroup -g 1000 -S invidious && \

docker/Dockerfile.arm64

@@ -1,4 +1,4 @@
-FROM alpine:3.20 AS builder
+FROM alpine:3.21 AS builder
 RUN apk add --no-cache 'crystal=1.12.2-r0' shards sqlite-static yaml-static yaml-dev libxml2-static \
        zlib-static openssl-libs-static openssl-dev musl-dev xz-static
 
@@ -33,7 +33,7 @@ RUN --mount=type=cache,target=/root/.cache/crystal if [[ "${release}" == 1 ]] ;
         --link-flags "-lxml2 -llzma"; \
     fi
 
-FROM alpine:3.20
+FROM alpine:3.21
 RUN apk add --no-cache rsvg-convert ttf-opensans tini tzdata
 WORKDIR /invidious
 RUN addgroup -g 1000 -S invidious && \