pin the base image tag for compatibility; allow host editing of uploaded files

[alfredp]

Problem: latest Alpine is missing DBM libraries used by WebDAV module

The Dockerfile specifies httpd:alpine as a base, however when I build an image locally, run it, and test file upload, an error occurs similar to the issue described here: docker-library/httpd#201 (comment)

Solution: use a Docker image tag that includes the required libs

Pin the base image to httpd:2.4.43-alpine which is the most recent tag that uses Alpine 3.12, which includes the required DBM libraries.

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Problem: files uploaded via WebDAV cannot be modified by the host

Upon startup, the entrypoint script changes ownership of all files and directories beneath and including the /var/lib/dav mount to the container user www-data. Newly created files are also owned by this user, and thus cannot be removed or modified by a non-root user on the host.

Solution: allow host edits by using an optional environment variable

Introduce an optional environment variable LOCAL that changes the umask for newly created files and directories that allows edits by a host user, by default.