bake: initial set of composable bake attributes

This allows using either the csv syntax or object syntax to specify
certain attributes.

This applies to the following fields:
- output
- cache-from
- cache-to
- secret
- ssh

There are still some remaining fields to translate. Specifically
ulimits, annotations, and attest.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>

bake/hclparser/gohcl/doc.go

@@ -0,0 +1,65 @@
+// Copyright (c) HashiCorp, Inc.
+// SPDX-License-Identifier: MPL-2.0
+
+// Package gohcl allows decoding HCL configurations into Go data structures.
+//
+// It provides a convenient and concise way of describing the schema for
+// configuration and then accessing the resulting data via native Go
+// types.
+//
+// A struct field tag scheme is used, similar to other decoding and
+// unmarshalling libraries. The tags are formatted as in the following example:
+//
+//	ThingType string `hcl:"thing_type,attr"`
+//
+// Within each tag there are two comma-separated tokens. The first is the
+// name of the corresponding construct in configuration, while the second
+// is a keyword giving the kind of construct expected. The following
+// kind keywords are supported:
+//
+//	attr (the default) indicates that the value is to be populated from an attribute
+//	block indicates that the value is to populated from a block
+//	label indicates that the value is to populated from a block label
+//	optional is the same as attr, but the field is optional
+//	remain indicates that the value is to be populated from the remaining body after populating other fields
+//
+// "attr" fields may either be of type *hcl.Expression, in which case the raw
+// expression is assigned, or of any type accepted by gocty, in which case
+// gocty will be used to assign the value to a native Go type.
+//
+// "block" fields may be a struct that recursively uses the same tags, or a
+// slice of such structs, in which case multiple blocks of the corresponding
+// type are decoded into the slice.
+//
+// "body" can be placed on a single field of type hcl.Body to capture
+// the full hcl.Body that was decoded for a block. This does not allow leftover
+// values like "remain", so a decoding error will still be returned if leftover
+// fields are given. If you want to capture the decoding body PLUS leftover
+// fields, you must specify a "remain" field as well to prevent errors. The
+// body field and the remain field will both contain the leftover fields.
+//
+// "label" fields are considered only in a struct used as the type of a field
+// marked as "block", and are used sequentially to capture the labels of
+// the blocks being decoded. In this case, the name token is used only as
+// an identifier for the label in diagnostic messages.
+//
+// "optional" fields behave like "attr" fields, but they are optional
+// and will not give parsing errors if they are missing.
+//
+// "remain" can be placed on a single field that may be either of type
+// hcl.Body or hcl.Attributes, in which case any remaining body content is
+// placed into this field for delayed processing. If no "remain" field is
+// present then any attributes or blocks not matched by another valid tag
+// will cause an error diagnostic.
+//
+// Only a subset of this tagging/typing vocabulary is supported for the
+// "Encode" family of functions. See the EncodeIntoBody docs for full details
+// on the constraints there.
+//
+// Broadly-speaking this package deals with two types of error. The first is
+// errors in the configuration itself, which are returned as diagnostics
+// written with the configuration author as the target audience. The second
+// is bugs in the calling program, such as invalid struct tags, which are
+// surfaced via panics since there can be no useful runtime handling of such
+// errors and they should certainly not be returned to the user as diagnostics.
+package gohcl