Supported Platform.sh provided environmental vars, fixes #82

[gilzow]

Closes #82
Adds a list of Platform-provided environmental variables that are currently supported in DDEV.

List of variables that are "missing" or not yet implemented:

• PLATFORM_APPLICATION
• PLATFORM_BRANCH
• PLATFORM_DOCUMENT_ROOT
• PLATFORM_ENVIRONMENT_TYPE
• PLATFORM_OUTPUT_DIR
• PLATFORM_SMTP_HOST
• PLATFORM_SOURCE_DIR
• PORT - not always available
• SOCKET

There are two PLATFORM_* environmental variables in DDEV that do not map to ones we provide.

• PLATFORM_DIR - is this possibly supposed to be PLATFORM_DOCUMENT_ROOT that is "missing"?
• PLATFORM_MOUNTS

[rfay]

@gilzow

• if PLATFORM_DIR is not an appropriate thing, it should be removed. I don't see any usages of it.
• If PLATFORM_MOUNTS is not used by platform, it should be removed. It's not used by code in this add-on, so it must be a redirection of some type.

[rfay]

This seems right. If we want to also remove the orphaned env vars you point out that is also reasonable, if we're sure they're not used by Platform.sh

[gilzow]

@rfay I sent you a direct message, re: PLATFORM_DIR

I've never seen PLATFORM_MOUNTS and it isn't in the docs. I've always accessed mount information via
$ echo $PLATFORM_APPLICATION | base64 --decode | jq -r '.mounts'

Looks like it was added here?

I guess we could rename it to DDEV_PLATFORM_MOUNTS just to designate that it's a DDEV env var related to PLATFORM, vs an "official" PLATFORM_* env var?

[rfay]

Followup in

• Remove PLATFORM_MOUNTS and PLATFORM_DIR #88