feat(SealedSecret pipeline): read cert from env or volumeMounts

[kirederik]

the current sealedsecret image uses kubectl to read a configmap with a known name to find out the certificate to use;

a better approach would to support reading the certificate from env or from a file within the local system; this would allow penny to use env or volumeMounts on her pipeline spec. This would also making reading the configmap with a known name redundant, so we can remove it.

tasks

• support for getting the certificate via a environment variable
• support for getting the certificate from a volumeMount
• remove reading the configmap with a known name
• update docs
• bump the version and release a new image