Update base image to resolve vulnerabilities

[claussen]

What:
update base version to fix vulns described in a docker scan eknert/nginx-tls-terminator

Why:
the current version does not pass a docker scan, due to vulnerabilities in the base image.

Testing:

1. build image with new Dockerfile
2. docker tag {container ID} eknert/nginx-tls-terminator:1.21.5-alpine
3. docker scan -f ./Dockerfile eknert/nginx-tls-terminator:1.21.5-alpine

Testing eknert/nginx-tls-terminator:1.21.5-alpine...

Package manager:   apk
Target file:       ./Dockerfile
Project name:      docker-image|eknert/nginx-tls-terminator
Docker image:      eknert/nginx-tls-terminator:1.21.5-alpine
Platform:          linux/amd64
Base image:        nginx:1.21.5-alpine

✔ Tested 43 dependencies for known vulnerabilities, no vulnerable paths found.

According to our scan, you are currently using the most secure version of the selected base image

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