[docker] Bump alpine from 3.20.3 to 3.21.0

Bumps alpine from 3.20.3 to 3.21.0.

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

Signed-off-by: dependabot[bot] <support@github.com>

Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.20.3 AS builder
+FROM alpine:3.21.0 AS builder
 
 COPY . /app
 
@@ -21,7 +21,7 @@ RUN COLOUR='\e[1;93m' && \
   pip install ./app && \
   echo -e "${COLOUR}Done.\e[0m"
 
-FROM alpine:3.20.3
+FROM alpine:3.21.0
 LABEL maintainer="Patrick Baus <patrick.baus@physik.tu-darmstadt.de>"
 LABEL description="Labnode PID controller daemon"