Add support for reading from docker secret files. New script prefers environment variables ending with _FILE over normal variables, meaning that Docker secrets will take precedence. You can, however, mix variable types, ex. MYSQL_USER uses a normal environment variable, while MYSQL_PASSWORD uses a secret.
There's no need to clean up the local Maven repository after the
build, since only the artifacts we want are copied to the runtime
image. For reasons not investigated, attempting to remove the
local repository directory fails because the directory is in use
after the build.
