Dockerfile Improvements

[aminvakil]

• Bump alpine 3.12.
• Use apk add --no-cache instead of apk update && apk install && rm -rf /var/cache/apk.
• https://github.com/docker/docker.github.io/blob/master/develop/develop-images/dockerfile_best-practices.md#using-pipes, since there are some shells that do not accept the -o pipefail option, it is not enough to add set -o pipefail inside the RUN instruction. Therefore, we recommend to always explicitly the SHELL before using pipes in RUN (https://github.com/hadolint/hadolint/wiki/DL4006#rationale).
• Quote to prevent word splitting.